The circumgalactic medium (CGM) plays a central role in galaxy evolution, acting as both the reservoir for gas accretion and the interface through which feedback regulates star formation. However, direct observational constraints on the spatial structure and physical conditions of the CGM remain limited. The low-redshift universe provides a unique opportunity in which host galaxy properties can be well constrained, and inflows and outflows can be resolved with sufficient spatial detail to directly probe the interaction between the CGM and the interstellar medium (ISM).
\r\n\r\nIn this thesis, I present a study of the ionized CGM using deep integral-field spectroscopy with the Keck Cosmic Web Imager (KCWI), based on a survey of 12 star-forming galaxies spanning nearly two orders of magnitude in stellar mass at z ~ 0.1. These observations enable sensitive mapping of faint emission lines such as H-alpha, [O III] 5007, and [O II] 3727, 3729, yielding spatially resolved measurements of CGM emission on scales of tens of kiloparsecs and opening a new window into the kinematics and ionization structure of diffuse halo gas.
\r\n\r\nA detailed case study of the low-mass galaxy J0910b reveals extended CGM emission with complex kinematics, including evidence for filamentary inflow and counter-rotation relative to the host galaxy. Emission-line ratios indicate that the gas is predominantly photoionized, consistent with a diffuse ultraviolet (UV) background and leakage of ionizing photons from star formation, rather than strong shock heating.
\r\n\r\nExtending this analysis to a broader sample, I identify a coherent scaling relation between CGM emission properties and galaxy stellar mass, defining a \u201cCGM main sequence.\u201d Across the sample, the radial extent and surface brightness of emission-line halos exhibit self-similar behavior when scaled by virial radius, with a clear transition near log(M_star/M_sun) ~ 10.5. This sequence indicates that the cool ionized CGM traces the underlying hot, diffuse halo and is linked to the dark matter potential.
\r\n\r\nIn this framework, a galaxy\u2019s position reflects the balance between gas accretion, star formation, and feedback over its assembly history: low-mass, actively star-forming galaxies occupy a bright, cool-gas-rich branch capable of sustaining continued growth, whereas higher-mass systems lie on a suppressed, baryon-poor branch shaped by strong feedback and virial heating as they transition toward quenching. This result establishes the first structural scaling law for the ionized CGM in individual low-redshift galaxies, in direct analogy to the star-formation main sequence.
\r\n\r\nOptical observations primarily probe the cool (~10^4 K) phase of the CGM. Key tracers of the warm-hot phase, such as Ly-alpha, C IV 1548, 1550, and O VI 1032, 1038, are shifted into the UV. To access this regime, our group has led the FIREBall-2 (Faint Intergalactic-medium Redshifted Emission Balloon) experiment, a suborbital UV spectroscopic mission designed to probe the multiphase structure of the CGM.
\r\n\r\nI contributed to the development and optimization of the FIREBall-2 UV detector system, including performance characterization of a photon-counting EMCCD during the 2022\u20132023 flight campaigns. Although no UV science data were obtained due to a balloon envelope leak, extensive pre-flight testing demonstrated the instrument performance and advanced its technology readiness level, paving the way for future UV missions probing the full multiphase CGM.
", "doi": "10.7907/b4ag-0q08", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17654", "collection": "thesis", "collection_id": "17654", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08272025-164020661", "primary_object_url": { "basename": "Sam_Ponnada_PhD_Thesis_Final.pdf", "content": "final", "filesize": 24112901, "license": "other", "mime_type": "application/pdf", "url": "/17654/2/Sam_Ponnada_PhD_Thesis_Final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "On the Non-Thermal Physics of Magnetic Fields and Cosmic Rays in Galactic Ecosystems", "author": [ { "family_name": "Ponnada", "given_name": "Sam Bharat Vijay K.", "orcid": "0000-0002-7484-2695", "clpid": "Ponnada-Sam-Bharat-Vijay-K" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Ravi", "given_name": "Vikram", "orcid": "0000-0002-7252-5485", "clpid": "Ravi-Vikram" }, { "family_name": "Bellan", "given_name": "Paul Murray", "orcid": "0000-0002-0886-8782", "clpid": "Bellan-P-M" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Most", "given_name": "Elias R.", "orcid": "0000-0002-0491-1210", "clpid": "Most-E-R" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The role of the non-thermal components of galaxies, magnetic fields B) and relativistic charged particles, known as cosmic rays (or CRs), is one of the most uncertain aspects of our understanding of galaxy formation and evolution. While magnetic fields and cosmic rays have long been known to be important components of our own Galaxy, the Milky Way, their part in shaping galactic ecosystems remains elusive. This owes partly to fundamentally indirect observations of physical quantities relevant to B and CRs which are fraught with questionable assumptions to make any physical inference, and partly due to the difficulty in modeling them theoretically.
\r\n\r\nIt has only become possible in the past decade to fully model B and CRs dynamically in simulations of galaxy formation within a cosmological context, all while maintaining high hydrodynamic resolution and evolving the relatively well-constrained physics of star formation and stellar feedback to produce realistic bulk- and spatially-resolved galaxy properties without calibration. In this thesis, I use state-of-the-art simulations which explicitly evolve B and CRs in concert with these explicit treatments of star formation and stellar feedback in a cosmological context towards two ends. One is to better understand where our observational assumptions oft used in our indirect constraints may go awry and to develop more physical estimators of B and CRs. In Chapters 2 and 3, I explore this avenue using by generating a host of synthetic observations. The second is to hold the well-constrained (to within an order-of-magnitude) physics fixed, and explore much more widely uncertain physics, namely that of cosmic ray transport, to constrain via emergent observables. In Chapters 4 and 5, I generate synthetic observational predictions across the electromagnetic spectrum for simulations with orders-of-magnitude variation in cosmic ray transport and compare to observations.
\r\n\r\nIn Chapter 6, I develop a novel analytic framework to survey the vastly uncertain CR transport parameter space, and explore implications for arbitrarily complex injections of cosmic rays from episodic black hole accretion or star formation, and outline a sub-grid model to incorporate CRs in large volume cosmological simulations which otherwise would suffer from additional computational overhead or artefacts arising from time-independent modeling assumptions. Finally, I conclude summarizing the constraints these various studies provide on galactic B and CRs, and the outlook for future simulations and observational comparisons.
", "doi": "10.7907/hjjy-rq17", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17454", "collection": "thesis", "collection_id": "17454", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06122025-215511097", "primary_object_url": { "basename": "Caltech_Thesis_LaTeX_Template__with_logo_.pdf", "content": "final", "filesize": 26583303, "license": "other", "mime_type": "application/pdf", "url": "/17454/1/Caltech_Thesis_LaTeX_Template__with_logo_.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Expanding the Landscape of Tidal Disruption Events", "author": [ { "family_name": "Somalwar", "given_name": "Jean J.", "orcid": "0000-0001-8426-5732", "clpid": "Somalwar-Jean-J" } ], "thesis_advisor": [ { "family_name": "Ravi", "given_name": "Vikram", "orcid": "0000-0002-7252-5485", "clpid": "Ravi-Vikram" } ], "thesis_committee": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" }, { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "orcid": "0000-0001-5390-8563", "clpid": "Kulkarni-S-R" }, { "family_name": "Ravi", "given_name": "Vikram", "orcid": "0000-0002-7252-5485", "clpid": "Ravi-Vikram" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Massive black holes (black hole masses MBH \u2273 100 M\u2609) are key to our understanding of galaxy evolution, reionization, and physics that is as fundamental as general relativity. Many open questions remain surrounding MBH formation, growth, and demographics. We require methods of discovering BHs, as well as probes of accretion, that expand beyond analyses of the most luminous, accreting MBHs. Tidal disruption events (TDEs) occur when a star enters an orbit around an MBH with pericentric distance small enough that tidal forces from the MBH overcome the stellar self-gravity. They produce accretion flares on human timescales, lighting up otherwise quiescent MBHs and enabling direct observations of a newly formed accretion disk. Efforts to identify TDEs in optical, X-ray, and infrared survey data have led to the first populations of such events, but selection effects may be limiting our view of the TDE landscape; for example, we may be missing the lowest mass black holes or populations with certain types of host galaxies.
\r\n\r\nIn my thesis, I expand our understanding of the TDE landscape using novel TDE discovery methods. The first part of my thesis discusses eight examples of radio-selected TDEs with a range of multiwavelength properties. The first event was a long-lived, jetted TDE candidates, with evidence for ongoing energy injection into the jet despite a highly sub-Eddington accretion state, as has been observed in X-ray binary systems. The second event was an infrared, and radio bright TDE, which, despite showing no optical flare, can be modeled as a TDE analogous to optically-selected event but with significantly more dust and gas in the circumnuclear medium. I identified the final six events as radio-selected, optically-detected TDEs. I show that these events are largely analogous to the optically-selected events, despite being identified in a search with largely different selection effects.
\r\n\r\nIn the second part of my thesis, I expand to optical searches for nuclear transients, and I present two events that were identified in optical TDE searches aimed at identifying events that are excluded by typical selection criteria. The first is a candidate repeating, partial TDE, where a star is grazing the tidal radius on successive orbits and a small fraction of its mass is accreted. This event shows a fast-cooling, fast-evolving optical flare, unlike previously identified TDEs. Finally, I present a candidate accretion event onto an IMBH, identified in a search for hostless TDEs in optical data. My work shows the significant potential of current and upcoming surveys to identify flaring and variable MBHs in new regimes.
", "doi": "10.7907/ef07-a146", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18705", "collection": "thesis", "collection_id": "18705", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292026-230010965", "primary_object_url": { "basename": "dunne_delaney_2026.pdf", "content": "final", "filesize": 7138605, "license": "other", "mime_type": "application/pdf", "url": "/18705/3/dunne_delaney_2026.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Joint Analysis Techniques for Line Intensity Mapping, with\r\nApplication to the CO Mapping Array Project", "author": [ { "family_name": "Dunne", "given_name": "Delaney Anne", "orcid": "0000-0002-5223-8315", "clpid": "Dunner-Delaney-Anne" } ], "thesis_advisor": [ { "family_name": "Cleary", "given_name": "Kieran A.", "orcid": "0000-0002-8214-8265", "clpid": "Cleary-K-A" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" } ], "thesis_committee": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Bock", "given_name": "James J.", "orcid": "0000-0002-5710-5212", "clpid": "Bock-J-J" }, { "family_name": "El-Badry", "given_name": "Kareem J.", "orcid": "0000-0002-6871-1752", "clpid": "El-Badry-K-J" }, { "family_name": "Cleary", "given_name": "Kieran A.", "orcid": "0000-0002-8214-8265", "clpid": "Cleary-K-A" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Line intensity mapping (LIM) is an emerging observational technique with broad applications in cosmology and galaxy evolution. Rather than cataloging galaxies as discrete objects, LIM experiments measure the spatial fluctuations of galaxy populations across large three-dimensional volumes. Thus, LIM is able to measure the contribution to the overall spectral-line luminosity from all galaxies in a population, even the very faintest. LIM began with the 21 cm neutral hydrogen line but has recently been expanding to other tracers, such as CO, [CII], H\u03b1, and Ly\u03b1. The first generations of dedicated experiments targeting these tracers are beginning to come online. In order to draw science conclusions from their observations, these experiments must address challenges in the form of sensitivity and systematic errors.
\r\n\r\nIn this thesis, I develop an analysis technique designed to mitigate these challenges by stacking LIM data on the positions of galaxies in an external galaxy catalog. I use a custom-designed simulation pipeline to produce simultaneous mock observations of a LIM experiment and galaxy survey covering the same galaxy population, and use these simulations to build a theoretical understanding of this stacking statistic. I find that cosmological clustering of galaxies around the objects in the reference galaxy catalog drives the stacked signal.
\r\n\r\nI also use the stack to draw conclusions about molecular gas in the period preceding the peak of cosmic star formation, using data from the CO Mapping Array Project (COMAP), the leading CO LIM experiment. I use eBOSS and DESI quasars, as well as Ly\u03b1 emitters from HETDEX, as external reference catalogs. I do not detect any cosmic CO emission in the COMAP stacks, and I find a moderate tension between the resulting upper limits and the brightest models of the stacked emission. This tension could be attributable to non-linear radiative transfer or clustering properties of the galaxy catalogs, but it could also be due to models for cosmic CO overpredicting the average CO luminosity z~3.
\r\n\r\nFinally, I search for spectral-line emission from axionic dark matter by stacking the DESI and HETDEX galaxies over a range of rest frequencies. I constrain the axion decay rate \u0393 and the axion-photon coupling constant |ga\u03b3\u03b3|. In the axion mass range probed by COMAP, I establish the tightest constraints available on an axion model of dark matter.
", "doi": "10.7907/r11t-4p78", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18644", "collection": "thesis", "collection_id": "18644", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262026-233047065", "primary_object_url": { "basename": "prusinski_nikolaus_2026.pdf", "content": "final", "filesize": 17062665, "license": "other", "mime_type": "application/pdf", "url": "/18644/2/prusinski_nikolaus_2026.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Geometry and Kinematics of Baryonic Feedback", "author": [ { "family_name": "Prusinski", "given_name": "Nikolaus Zen", "orcid": "0000-0001-5847-7934", "clpid": "Prusinski-Nikolaus-Zen" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" } ], "thesis_committee": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "orcid": "0000-0001-8638-0320", "clpid": "Hillenbrand-L-A" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "This thesis addresses the connection between the physics of galactic-scale baryonic feedback, the morphology and kinematics of gas in the multiphase circumgalactic medium (CGM), and host galaxy properties. Broadly speaking, each of the projects in this thesis attempts (through different datasets and techniques) to quantify how star formation is regulated in galaxies; the relevant observable being the gas and metal distributions in and around galaxies. The geometry and kinematics of interstellar, circumgalactic, and intergalactic gas provide insight into the efficacy and timescales over which galaxy-driven baryonic feedback processes operate.
\r\n\r\nChapter 2 features the highest fidelity maps of the average z ~ 2 CGM to date using foreground/background galaxy pairs with angular separations < 30\" in the Keck Baryonic Structure Survey (KBSS). The maps of covering fraction vs. transverse distance and line-of-sight velocity chart the changing gravitational influence the galaxy has over its CGM. Ly-alpha absorption shows a distinct transition near the virial radius of the average foreground galaxy and overall, dark matter halo mass appears to strongly modulate the kinematics, covering fraction, and spatial extent of CGM gas, a significant fraction of which may be unbound.
\r\n\r\nChapter 3 presents new results on the spatial distribution and kinematics of nonresonant emission in and around z ~ 2 KBSS galaxies observed with the Keck Cosmic Web Imager (KCWI). Nonresonant Si II* emission lines have velocity spreads most consistent with a zero-velocity offset \"ISM\" component fit to their corresponding resonant Si II absorption lines. Extended Si II* halos are observed for the first time at z ~ 2 with a decreasing velocity spread with increasing transverse distance. At large radii, photon conservation appears to hold in that the flux \"removed\" by resonant absorption approximately equals the flux contributed by nonresonant emission.
\r\n\r\nChapter 4 outlines the data reduction procedures needed to produce the integral field spectra used in this thesis. The KCWI data reduction pipeline stages I have improved or still need improvement are discussed. I introduce KCWIKit, a toolkit for mosaicking and correcting common systematics in integral field unit datacubes.
\r\n\r\nFinally, Chapter 5 introduces two ongoing projects considering how supermassive black holes couple to their host galaxies, their CGM, and intergalactic media (IGM) via Mpc-scale jets. Using KCWI, I am investigating the driving mechanisms behind Mpc-scale jets and searching for collisionally-excited emission from the giant jet bow shock. Such a detection would be the first detection of the warm-hot IGM (WHIM) in rest-frame UV emission and would open up a new means of mapping the distribution of metals in the IGM. The latter remains a critical frontier in accounting for the baryons in the universe and understanding the large-scale effects of galactic feedback.
", "doi": "10.7907/gnqa-tm83", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17708", "collection": "thesis", "collection_id": "17708", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09302025-183958240", "primary_object_url": { "basename": "CarvalhoCaltechThesisDocument.pdf", "content": "final", "filesize": 86620807, "license": "other", "mime_type": "application/pdf", "url": "/17708/1/CarvalhoCaltechThesisDocument.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "A Detailed Study of the Inner Disks of FU Ori Objects and the Star-Disk Boundary", "author": [ { "family_name": "Carvalho", "given_name": "Adolfo Sjoberg de", "orcid": "0000-0002-9540-853X", "clpid": "Carvalho-Adolfo-Sjoberg-de" } ], "thesis_advisor": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "orcid": "0000-0001-8638-0320", "clpid": "Hillenbrand-L-A" } ], "thesis_committee": [ { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Batygin", "given_name": "Konstantin", "orcid": "0000-0002-7094-7908", "clpid": "Batygin-K" }, { "family_name": "Blake", "given_name": "Geoffrey A.", "orcid": "0000-0003-0787-1610", "clpid": "Blake-G-A" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Hillenbrand", "given_name": "Lynne A.", "orcid": "0000-0001-8638-0320", "clpid": "Hillenbrand-L-A" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "FUOrs are young stellar objects (YSOs) whose disks undergo an instability that raises the disk-to-star accretion rate by a factor of 1,000 to 10,000, and sustain this elevated accretion rate for up to 100s of years. The outbursts may be a phase through which all young low-mass stars pass in the early stages of their formation and could have played a role in the thermal processing of material in the Solar System. In this thesis, I present a thorough and comprehensive study of the inner disks of FUOrs and the properties of the accreting stars themselves.
\r\n\r\nIn Chapter I, I contextualize FUOr accretion disks in the timeline of pre-main sequence stellar evolution and discuss some of the major open questions in the study of FUOrs today. I address the mechanisms proposed to trigger FUOr outbursts and how the work in this thesis, as well as future observations, can constrain existing models for each trigger mechanism.
\r\n\r\nIn Chapter II, I introduce the disk modeling infrastructure that I use to study FUOrs and to constrain physical parameters like their masses and mass accretion rates. I include a tutorial of how different parameters in the accretion disk model impact the spectral energy distributions (SEDs) and high resolution spectra. Finally, I give a recommendation for a consistent approach to estimating and interpreting the luminosities of FUOrs and enumerate some important caveats to consider when using the accretion disk model in this Thesis.
\r\n\r\nIn Chapters III-VI, I rigorously test the accuracy of the accretion disk model with SEDs, and high and low resolution spectra, spanning visible to NIR wavelengths. The focus of Chapters III and IV is the photometric and spectroscopic evolution of V960 Mon following the first 3 years after the peak of its outburst. Disk model fitting indicates that the inner radius of the disk expanded as the post-peak accretion rate decreased. The rate of expansion of the inner radius is consistent with the magnetosphere re-establishing itself as the accretion flow weakens. Chapter V follows the evolution of HBC 722 in the first 10 years of its outburst. In this source, the size of the accretion-dominated inner disk grows over a 5 year period, indicating that the instability driving the outburst can ignite a larger region of the disk than where it began. In Chapter VI, I apply the disk model to V883 Ori, a FUOr with a rich molecular millimeter emission spectrum, to measure physical properties of its inner disk. I find that the visible light from the source is unexpectedly bright for its NIR-derived extinction value, but that the discrepancy can be explained by material near the source scattering light toward the observer along a less-extincted line of sight.
\r\n\r\nIn Chapter VII, I compute bolometric corrections for measuring the accretion luminosity of a FUOr from a single photometric observation. The bolometric corrections are tabulated for a broad range of accretion luminosities using the filters of 6 photometric surveys/observatories: ZTF, Gaia, LSST, 2MASS, Roman, and WISE. The work includes $A_V$ to in-band extinction calibrations for each filter assuming the underlying SED of a FUOr. The bolometric corrections agree with more rigorously-derived accretion luminosities of 4 FUOrs. They are useful when estimating the peak accretion luminosity of a FUOr given a measured value from one epoch and a lightcurve for the source, and for converting lightcurves to accretion rate evolution of a disk during its outburst.
\r\n\r\nIn Chapter VIII, I use cross-correlation functions (CCFs) of high resolution near-infrared spectra to compute the maximum rotational broadening in the disks of 20 FUOrs and to test a critical assumption of the accretion disk model. Measuring rotational broadening in FUOrs is challenging due to the differential rotation of the disk. I reveal a means of using CCFs of observed and modeled $H$ band spectra to measure the maximum rotational broadening in the disk while accounting for differential rotation. I then apply the technique to Keck/NIRSPEC spectra of 20 FUOrs to measure $v_\\mathrm{max}$ in the disks. I also compute the CCFs for Y, J, H, and K band spectra of FUOrs and find almost all show the expected differential rotation. I also compute the same CCFs for model spectra to show that the degree of observed differential rotation depends on the size of the active accretion region in the disk.
\r\n\r\nIn Chapter IX, I use the infrastructure presented in the previous Chapters to measure the physical parameters of 4 well-known FUOrs. Two of them, V1057 Cyg and V1515 Cyg, are among the first known FUOrs and have been studied since the 1970s. The third is BBW 76, one of the oldest known FUOrs, and the fourth is Gaia 17bpi, which began its outburst in 2017 and appears to be returning to its pre-outburst state.
\r\n\r\nIn Chapters X and XI, I present the first detection of the FUV continuum in FU Ori, as well as an NUV spectroscopic survey of 6 FUOrs. Using the FUV continuum flux of FU Ori and a newly-fit disk model, I demonstrated that the 40 year-old model for the star-disk interface at FU Ori cannot match the observed spectrum. I then applied the same technique to the bluest wavelengths of all 6 sources in the NUV survey and found that they have UV emission in excess of the disk model spectrum, but inconsistent with the emission predicted by the classical boundary layer model proposed for FUOrs.
\r\n\r\nChapters XII and XIII contain a high resolution spectral atlas of $15-20$ FUOrs using Keck/HIRES and Keck/NIRSPEC. In total, the atlas covers 0.47 to 2.40 microns at spectral resolutions of 15,000 (NIRSPEC) to 35,000 (HIRES). In the two chapters, I briefly compare the spectra of FUOrs at high resolution. The data from the atlases appear throughout the other chapters in this Thesis.
\r\n \r\nI conclude the Thesis with Chapter XIV, in which I summarize several upcoming observational technologies that, when combined with the data and models presented in this thesis, will produce a complete picture of the FUOr variable class.
", "doi": "10.7907/4e53-3n79", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17609", "collection": "thesis", "collection_id": "17609", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08112025-205331711", "primary_object_url": { "basename": "Thesis_Zhuyun_Zhuang.pdf", "content": "final", "filesize": 25014516, "license": "other", "mime_type": "application/pdf", "url": "/17609/3/Thesis_Zhuyun_Zhuang.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Multi-Element Abundances as Probes of Galaxy Growth Across Cosmic Time", "author": [ { "family_name": "Zhuang", "given_name": "Zhuyun", "orcid": "0000-0002-1945-2299", "clpid": "Zhuang-Zhuyun" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "thesis_committee": [ { "family_name": "Ravi", "given_name": "Vikram", "orcid": "0000-0002-7252-5485", "clpid": "Ravi-Vikram" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Kasliwal", "given_name": "Mansi M.", "orcid": "0000-0002-5619-4938", "clpid": "Kasliwal-Mansi-M" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "This thesis presents a comprehensive study of multi-elemental abundances in galaxies across a wide range of stellar masses, star formation rates, and redshifts, aiming to better characterize mass-metallicity relations (MZRs) and to probe the physical processes that govern chemical enrichment in galaxies over cosmic time.
\r\n\r\nChapter 2 demonstrates that low- and high-mass stellar MZRs can be placed on a consistent absolute scale by comparing stellar metallicities from resolved and integrated-light spectroscopy in NGC 147, a Local Group dwarf galaxy. The two methods yield consistent [Fe/H] values, validating integrated-light techniques in measuring stellar metallicity in distant galaxies.
\r\n\r\nIn Chapter 3, I analyze deep Keck spectra of two gravitationally lensed, quiescent galaxies at z ~ 1. Both galaxies are metal-poor compared to local analogs but share comparable [Mg/Fe]. These results suggest that both galaxies experienced early, rapid star formation and strong outflows that depleted their gas reservoirs and drove early quenching.
\r\n\r\nChapter 4 introduces a new sample of 46 star-forming and 2 quiescent systems with M* = 108–1010 M☉ at z ∼ 0, observed with the Keck Cosmic Web Imager (KCWI). Combining with SDSS data, I construct the first consistently derived Fe- and Mg-MZRs from 108 to 1011.5 M*. I find that scatter in the stellar MZRs is driven by varying sSFR, with more active galaxies showing lower stellar abundances at fixed mass. This chapter also presents the first comparison of α elements (O and Mg) in stars and the ionized gas beyond the Local Group, revealing a tight [Mg/H]*–[O/H]g correlation. The finding that [O/H]g generally exceeds [Mg/H]* implies star formation is fueled by enriched, well-mixed interstellar medium rather than pristine gas inflows.
\r\n\r\nChapter 5 further explores spatially resolved nebular and stellar properties in the KCWI sample. Star-forming dwarfs exhibit clumpy star formation, diverse local ionization conditions, and smooth stellar mass profiles. Some show established gas-phase metallicity gradients, although diffuse ionized gas may bias gradient measurements.
\r\n\r\nTogether, these studies advance our understanding of how elemental abundances evolve across different galaxy masses and redshifts, offering new insights into the processes that regulate galaxy growth, enrichment, and quenching.
", "doi": "10.7907/5e14-b360", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17238", "collection": "thesis", "collection_id": "17238", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05162025-170026255", "primary_object_url": { "basename": "Thesis (3).pdf", "content": "final", "filesize": 29172617, "license": "other", "mime_type": "application/pdf", "url": "/17238/1/Thesis (3).pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Dust in Astrophysical Systems: Impacts on Dynamics, Plasma Physics, and Thermochemistry", "author": [ { "family_name": "Soliman", "given_name": "Nadine Hany", "orcid": "0000-0002-6810-1110", "clpid": "Soliman-Nadine-Hany" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Phinney", "given_name": "E. Sterl", "orcid": "0000-0002-9656-4032", "clpid": "Phinney-E-S" }, { "family_name": "Hillenbrand", "given_name": "Lynne A.", "orcid": "0000-0001-8638-0320", "clpid": "Hillenbrand-L-A" }, { "family_name": "Batygin", "given_name": "Konstantin", "orcid": "0000-0002-7094-7908", "clpid": "Batygin-K" }, { "family_name": "Ravi", "given_name": "Vikram", "orcid": "0000-0002-7252-5485", "clpid": "Ravi-Vikram" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "The role of astrophysical dust is often simplified in models of galaxy and star formation, where it is typically treated as passively tracing the gas. However, dust actively influences the dynamics, thermodynamics, and observable properties of diverse environments. This thesis explores how explicitly modeling dust grain dynamics and their interactions with gas, radiation, and electromagnetic fields alters the behavior of three key astrophysical regimes: active galactic nuclei (AGN), star-forming giant molecular clouds (GMCs), and the early Universe.
\r\n\r\nRadiation pressure on dust is widely regarded as a driver of large-scale outflows in AGN, though the dynamics of this interaction remain poorly constrained. Using radiation\u2013dust\u2013magnetohydrodynamic (RDMHD) simulations, we show that radiation efficiently drives supersonic, dust-laden outflows, which are unstable to resonant drag instabilities (RDIs). These instabilities generate turbulence and restructure the dust into clumpy, anisotropic forms, accounting for the torus's patchiness and producing time-variable reprocessed emission in the infrared and optical.
\r\n\r\nIn GMCs, dust dynamics play a crucial role in shaping both the chemistry and thermodynamics of the gas. Leveraging the STARFORGE framework with live dust dynamics and non-equilibrium thermochemistry, we show that stellar radiation redistributes dust, reducing dust accretion during the main mass growth phases and leading to substantial abundance variations among co-natal stars. Statistically, these variations align with observational data, providing an alternative mechanism for driving abundance fluctuations in co-natal stars, beyond interpretations focused solely on post-formation processes like planet accretion. We find that, for a fixed dust mass, grain size variations significantly affect the thermodynamics, influencing local opacity, radiative transport, thermal balance, and ionization structure, thereby suppressing SFE by up to an order of magnitude.
\r\n\r\nFinally, we introduce a novel mechanism for magnetogenesis in the early Universe via radiatively accelerated, charged dust grains. This \"dust battery\" takes advantage of the large stopping lengths of dust grains to produce significant charge separation over large distances, thereby driving electric fields and seeding magnetic fields. Unlike conventional mechanisms (e.g., Biermann battery, Weibel instability), which rely on short-range electron-ion separation, this process operates efficiently and generates magnetic fields several orders of magnitude stronger than those produced by traditional mechanisms. We derive the underlying theory, develop a Magnetohydrodynamic-Particle-In-Cell (MHD-PIC) model, and a sub-grid model suitable for implementation in cosmological contexts.
\r\n\r\nThese insights underscore the importance of incorporating dust dynamics into astrophysical models to enhance our understanding of the formation and evolution of galaxies, stars, and the interstellar medium.
", "doi": "10.7907/qszw-vm92", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17287", "collection": "thesis", "collection_id": "17287", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282025-211817948", "primary_object_url": { "basename": "Nunez-Cravin_Thesis.pdf", "content": "final", "filesize": 9399953, "license": "other", "mime_type": "application/pdf", "url": "/17287/1/Nunez-Cravin_Thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "An \"InCLOSE\" View of the Circumgalactic Medium of z~2 Star-Forming Galaxies", "author": [ { "family_name": "Nunez-Cravin", "given_name": "Evan Haze", "orcid": "0000-0001-5595-757X", "clpid": "Nunez-Cravin-Evan-Haze" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "thesis_committee": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Kasliwal", "given_name": "Mansi M.", "orcid": "0000-0002-5619-4938", "clpid": "Kasliwal-Mansi-M" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "This thesis focuses on using diffuse gas to investigate the galactic chemical evolution and circumgalactic medium of galaxies near the peak of cosmic star-formation rate density z ~ 2. There are many fundamental questions that remain unanswered about these processes due to the lack of large observational samples including what the typical yields of massive stars are, the interplay between diffuse circumgalactic and dense interstellar gas in terms of kinematic complexity, metal content, stellar mass, and star formation rate, and the evolution of circumgalactic gas over cosmic time. The common thread between each investigation is the use of QSO absorption line objects (QSO absorbers) to probe diffuse gas that otherwise would be unseen due to its diffusivity.
\r\n\r\nThe chemical evolution of galaxies requires accounting for all sources of nucleosynthesis. During the earliest stages of galactic chemical evolution, the metal yields from core-collapse supernovae (CCSNe) are very important but acquiring empirical constraints is difficult because they cannot be easily disentangled from objects that currently exist because they have been enriched by some fraction of CCSNe and late time nucleosynthetic processes e.g., Type Ia SN. To address this, I used the metal abundances of very-metal poor (VMP; [Fe/H] <-2) Damped Lyman Alpha Absorbers (DLAs; QSO absorbers with high H I column density comparable to the interstellar medium, log(NHI/cm\u207b\u00b2 > 20.3) to place empirical constraints on the yields of low-metallicity CCSNe. I found that this approach is comparable to, and sometimes superior to, using abundances from the atmospheres of metal poor stars because of the model-independent nature of measuring abundances from dense, cold gas provided by the DLA.
\r\n \r\nIt has been known in the literature that DLAs and other QSO absorbers have a variety of origins so I began an observational campaign to find galaxies associated with QSO absorbers that would allow detailed analysis of the circumgalactic medium (CGM) of z ~ 2 galaxies. This has historically been challenging at all z, but especially at z ~ 2 where, before this thesis, there were only nine galaxies with their inner CGM analyzed (within a projected distance of 100 kpc) and with characterized nebular emission and stellar population properties.
\r\n\r\nTo this end, I am leading a survey that builds on the Keck Baryonic Structure Survey (KBSS) that aims to find close-in galaxy-QSO pairs to directly connect the Inner CGM of QSO Line Of Sight Emitting (InCLOSE) galaxies with their ISM. KBSS-InCLOSE relies on new observations that I have conducted using the twin Keck telescopes on Mauna Kea in Hawaii. I use the new optical integral field unit (rest-FUV at z ~ 2.3) called KCWI to discover new \"InCLOSE\" galaxies; obtain follow Keck/MOSFIRE near infrared (NIR) spectroscopy to confirm their redshifts and infer nebular properties including star-formation rate; use ground- and space-based optical and NIR images to infer stellar mass and age; and finally use high-resolution optical spectra of the KBSS QSOs to perform detailed analysis of CGM gas seen as absorption in the QSO spectra. The novelty of KBSS-InCLOSE goes beyond its large size (55 galaxies currently); the NIR spectra and images allow for the direct determination of galaxy properties, including stellar mass, which are rarely included in similar high-z surveys.
\r\n\r\nThe first results from KBSS-InCLOSE showcased the tools and techniques required to remove the bright QSOs from the datacubes, images, and spectra to reveal new faint, close-in galaxy-QSO pairs. Particular focus was payed on the processing of the IFU data because it serves as the main driving instrument for the survey since it provides both images and spectra of each galaxy in the field. By analyzing their CGM absorption, I showed that a M=M*=10\u00b9\u2070 M\u2609 z=2.43 galaxy exhibited strong, multiphase, kinematically complex, and gravitationally unbound metals in its CGM. This has been seen before in previous studies and may suggest that a consensus picture of the CGM of z ~ 2.3, M* galaxies is emerging.
\r\n\r\nIn KBSS-InCLOSE II, I focused on the first low-mass galaxies examined in the sample. I showed that the galaxies were star-forming, at the same redshift, and had sizes and masses consistent with dwarf galaxies, and found preliminary insights into the low-mass CGM that would make it distinct from both the massive $z\\sim2$ CGM and low-mass local CGM suggesting that there may be strong evolution of the CGM across both stellar mass and redshift.
\r\n\r\nIn Chapter \\ref{chapter5_InCLOSEIII} I preview work that is yet to be completed, KBSS-InCLOSE III, where I examined the entire sample showing that the z ~ 2 CGM often shows strong of metal absorption, is likely clumpy, and multiphase, and that future IFU follow-up is necessary to find more galaxies, and NIR spectroscopic follow-up is necessary to secure redshifts to mitigate mismatches between galaxy's and absorbers.
\r\n\r\nAltogether, this thesis has laid fundamental groundwork towards expanding our understanding of the galaxy-scale baryon cycle of z ~ 2 star-forming galaxies by building the largest z ~ 2 close-in galaxy-QSO pair observational dataset thus far. It provides the data required to perform the most detailed examination of the connection between galaxies and their CGM during the peak epoch for galaxy formation.
", "doi": "10.7907/0tj7-d742", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16470", "collection": "thesis", "collection_id": "16470", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022024-155428132", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 12568622, "license": "other", "mime_type": "application/pdf", "url": "/16470/1/Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Dissipative Dynamics of Stars, Planets, and Black Holes", "author": [ { "family_name": "Ma", "given_name": "Linhao", "orcid": "0000-0001-6117-5750", "clpid": "Ma-Linhao" } ], "thesis_advisor": [ { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" } ], "thesis_committee": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Howard", "given_name": "Andrew W.", "orcid": "0000-0001-8638-0320", "clpid": "Howard-Andrew-W" }, { "family_name": "Phinney", "given_name": "E. Sterl", "orcid": "0000-0002-9656-4032", "clpid": "Phinney-E-S" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "In this dissertation, I present a series of theoretical works on two important dissipative mechanisms in the universe, namely dynamical friction and tidal dissipation. I discuss the physics of these processes, and investigate how they will affect the dynamical evolution of stars, planets, and black holes.
\r\n\r\nI develop a new sub-grid dynamical friction estimator based on the discrete nature of N-body simulations. This estimator avoids the ambiguously defined quantities in Chandrasekhar's dynamical friction formula. I test the estimator in the GIZMO code, and find that it agrees well with high-resolution simulations where dynamical friction is fully captured. The additional computational cost with this estimator is negligible, making it an efficient and implementable solution to sub-grid dynamical friction modeling.
\r\n\r\nI study the dynamics of massive black hole seeds in high-redshift galaxies. I analyze the direct N-body integration of seed black hole trajectories with high-resolution cosmological simulations, and calculate the dynamics of randomly generated test particles in post-processing with dynamical friction. I find that seed black holes less massive than 100 million solar-masses (i.e. all but the already-supermassive seeds) cannot efficiently sink to the galactic center in typical high-redshift galaxies. This finding provides new constraints on the formation models of super-massive black holes in the most distant galaxies.
\r\n\r\nI study the effects of tidal resonance locking for exoplanet systems, in which the planet locks into resonance with a tidally excited stellar gravity mode. I find that due to nonlinear mode damping, resonance locking in Sun-like stars likely only operates for low-mass planets, but in stars with convective cores it can likely operate for all planetary masses. The orbital decay timescale with resonance locking is typically comparable to the star's main-sequence lifetime, corresponding to a wide range in effective stellar quality factor, depending on the planet's mass and orbital period. I make predictions for several individual systems and examine the orbital evolution resulting from both resonance locking and nonlinear wave dissipation.
\r\n\r\nI investigate the tidal spin-up of subdwarf B (sdB) star binaries. I directly calculate the tidal excitation of internal gravity waves in realistic sdB stellar models, and integrate the coupled spin-orbit evolution of sdB binaries. I find that for canonical sdB binaries, the transitional orbital period below which they could reach tidal synchronization in the sdB lifetime is approximately 0.2 days, with weak dependence on the companion masses. This value is very similar to the tidal synchronization boundary evident from observations.
\r\n\r\nI investigate the scenario of tidal spin-up of Wolf-Rayet-black-hole binaries, which is a possible way to form the fast-rotating black holes observed from gravitational wave events. I directly calculate the tidal excitation of oscillation modes in Wolf-Rayet star models, determining the tidal spin-up rate, and integrating the coupled spin-orbit evolution for Wolf-Rayet-black-hole binaries. I find that for short-period orbits and massive Wolf-Rayet stars, the tidal interaction is mostly contributed by standing gravity modes, in contrast to Zahn's model of traveling waves which is frequently assumed in the literature. I show that tidal synchronization is rarely reached in Wolf-Rayet-black-hole binaries, and the resulting black hole spins are less than 0.4 for all but the shortest period binaries.
", "doi": "10.7907/9j1c-ec50", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16498", "collection": "thesis", "collection_id": "16498", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06042024-015417159", "primary_object_url": { "basename": "Zhihui_Li_thesis.pdf", "content": "final", "filesize": 15317461, "license": "other", "mime_type": "application/pdf", "url": "/16498/1/Zhihui_Li_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Unveiling the Structure of the Circumgalactic Medium of High-Redshift Galaxies via Emission and Absorption Lines", "author": [ { "family_name": "Li", "given_name": "Zhihui", "orcid": "0000-0001-5113-7558", "clpid": "Loi-Zhihui" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "thesis_committee": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "orcid": "0000-0001-8638-0320", "clpid": "Hillenbrand-L-A" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "El-Badry", "given_name": "Kareem J.", "orcid": "0000-0002-6871-1752", "clpid": "El-Badry-K-J" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "The circumgalactic medium (CGM), namely the gaseous matter beyond the stars and interstellar medium of a galaxy and within the virial radius of its dark matter halo, plays a pivotal role in governing crucial aspects of galaxy evolution. This thesis focuses on investigating the multiphase, clumpy structure of the CGM through the application of state-of-the-art numerical simulations, the development of novel semi-analytic models, and comprehensive analyses involving comparison with high-resolution, spatially resolved observational data obtained from the world's largest ground-based telescopes.
\r\n \r\nIn this thesis, Chapter 2 represents a theoretical investigation into the fate of cool clouds within a hot ambient medium, which offers new insights into predicting the destiny of cool clouds based on observed CGM properties. Chapters 3 and 4 detail endeavors to model spatially resolved Ly\u03b1 emission spectra obtained from SSA22 Ly\u03b1 Blob 1 and 2 to constrain the cool gas properties, employing a multiphase, clumpy radiative transfer (RT) model for the CGM. Chapter 5 offers a theoretical exploration on extracting and interpreting physical parameters of cool gas in the CGM from Ly\u03b1 spectra using physically realistic RT models. Chapter 6 introduces a novel method to self-consistently reproduce the spatially extended Ly\u03b1 emission from the CGM of twelve extreme emission line galaxies at z ~ 2, marking the first successful attempt to model spatially varying Ly\u03b1 emission within a physically realistic CGM framework. Chapter 7 introduces ALPACA, a new semi-analytic model for simulating low-ionization state (LIS) metal absorption lines in the clumpy CGM. Applying ALPACA to model CII\u03bb1334 absorption line profiles in star-forming galaxies at 2 < z < 3, the study reveals the intricate physical and kinematic structure of the CGM, and it underscores the necessity of integrating emission and absorption line modeling to effectively break the intrinsic degeneracy of complex CGM models. Concluding the thesis, Chapter 8 offers a brief summary and outlines potential applications of the newly developed CGM models in the James Webb Space Telescope (JWST) era.
", "doi": "10.7907/42cq-6x31", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16393", "collection": "thesis", "collection_id": "16393", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05202024-175837810", "primary_object_url": { "basename": "samantha_c_wu_thesis_pdf.pdf", "content": "final", "filesize": 4424293, "license": "other", "mime_type": "application/pdf", "url": "/16393/2/samantha_c_wu_thesis_pdf.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "The Interplay of Waves and Stellar Evolution", "author": [ { "family_name": "Wu", "given_name": "Samantha Chloe", "orcid": "0000-0003-2872-5153", "clpid": "Wu-Samantha-Chloe" } ], "thesis_advisor": [ { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" } ], "thesis_committee": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Batygin", "given_name": "Konstantin", "orcid": "0000-0002-7094-7908", "clpid": "Batygin-K" }, { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" }, { "family_name": "Kasliwal", "given_name": "Mansi M.", "orcid": "0000-0002-5619-4938", "clpid": "Kasliwal-Mansi-M" }, { "family_name": "Phinney", "given_name": "E. Sterl", "orcid": "0000-0002-9656-4032", "clpid": "Phinney-E-S" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "In this thesis, I study the evolution of stellar interiors and stellar oscillations in order to address current observational puzzles in astronomy. The thesis focuses on refining physical models for pre-supernova outbursts from massive stars, tidal evolution of planetary architectures, and progenitors of compact neutron star binaries. In order to research these topics, I combine calculations of internal stellar oscillation modes with stellar evolution models, to account for the evolution of the modes across the stars' lives. I begin by introducing some concepts in stellar evolution and internal stellar oscillations which underlie the physical intuition and models present throughout the thesis.
\r\n\r\nIn the second chapter of the thesis, I present a study of tidal dissipation in M-dwarfs hosting nearby exoplanets. I model dynamical tides from normal modes of stellar oscillation across stellar evolution. With my novel methods, I am able to resolve the detailed spectrum of tidal dissipation as a function of stellar age. This empowers our evolutionary calculations to capture the resonance locking phenomenon, in which sustained tidal excitation of modes to large amplitudes over long intervals of stellar evolution produces enhanced dissipation. I find that Earth-mass and Jupiter-mass planets around M-dwarfs experience significant orbital migration under the influence of resonance locking with inertial modes of the star.
\r\n\r\nIn the third and fourth chapters of the thesis, I explore the ability of waves generated by vigorous core convection in massive stars to impart heat to the stellar envelopes. I model the excitation and propagation of waves during phases of energetic nuclear burning in stars to assess the amount of energy that waves transmit to the envelope, as well as the timescale before core collapse when the majority of wave heating occurs. I find that wave heating is unlikely to independently produce very massive circumstellar material (CSM), but induces large expansion that could trigger interaction with a binary companion and thereby drive the intense mass loss that is expected to precede interacting supernovae.
\r\n \r\nThe fifth chapter explores this very mechanism of binary interaction to produce pre-supernova outbursts. Even without wave heating, stripped stars can expand greatly in the years before core collapse simply as a consequence of stellar evolution. I employ binary stellar evolution simulations to study how stripped stars of around 2-3 solar masses interact with neutron star companions, crucially focusing on the often-omitted stages from oxygen/neon (O/Ne) burning onward. I observe the stripped stars to undergo extremely high rates of mass loss, which can form a distribution of dense CSM around the system. My estimates for the CSM mass and radius are consistent with observed low ejecta-mass, interacting supernovae.
", "doi": "10.7907/kmc6-kn66", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:15151", "collection": "thesis", "collection_id": "15151", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05052023-033943351", "type": "thesis", "title": "Mock Observations of the Sunyaev-Zel\u2019dovich Effect in Massive Galaxy Clusters and a Six-Layer Integral Antireflective Structure for Silicon Optics", "author": [ { "family_name": "Macioce", "given_name": "Theodore Kenneth", "orcid": "0000-0002-3156-6627", "clpid": "Macioce-Theodore-Kenneth" } ], "thesis_advisor": [ { "family_name": "Golwala", "given_name": "Sunil", "orcid": "0000-0002-1098-7174", "clpid": "Golwala-S-R" } ], "thesis_committee": [ { "family_name": "Zmuidzinas", "given_name": "Jonas", "orcid": "0000-0002-3330-5439", "clpid": "Zmuidzinas-J" }, { "family_name": "Golwala", "given_name": "Sunil", "orcid": "0000-0002-1098-7174", "clpid": "Golwala-S-R" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Faraon", "given_name": "Andrei", "orcid": "0000-0002-8141-391X", "clpid": "Faraon-A" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Part 1: Measuring the kinematic Sunyaev-Zel\u2019dovich (kSZ) effect is a promising observational tool to constrain both cosmic growth and galaxy cluster formation. As millimeter-wave telescopes gain sensitivity and angular resolution over multiple frequency bands, high signal-to-noise imaging of the kSZ effect in large samples of galaxy clusters will become increasingly feasible. However, maximizing the science reach of these upcoming data will require more sophisticated analysis methods to characterize and remove contamination from a range of unwanted signals, such as the emission from dusty star forming galaxies. Current predictions of kSZ-derived constraints do not account for these effects in sufficient detail. Moreover, they typically rely on Fisher matrix analyses, which cannot fully capture the degeneracies among the physical parameters describing the cluster. We present a mock observation and analysis pipeline to determine the science reach of kSZ galaxy cluster observations that employs more detailed noise models and more sophisticated analysis methods. From our mock observations, we derive new forecasts of the constraining power of next-generation telescopes on cluster peculiar velocities for several instrument configurations from the 10-m, 30-m, and 50-m classes. These forecasts will inform the designs of next-generation telescopes targeting kSZ observations and will indicate the optimal instrumentation for both cosmological and cluster-scale constraints. The software pipeline we develop will also be directly usable as an analysis tool once observations from such telescopes become available.
\r\n\r\nPart 2: Silicon optics can greatly benefit future millimeter and submillimeter astronomical instruments thanks to silicon\u2019s useful properties such as low loss, high refractive index, and high strength. However, silicon\u2019s high index (n = 3.4) necessitates antireflection (AR) treatment, which has proven a major challenge, especially for the multilayer treatments required for wide spectral bandwidths. We present our approach to this challenge, in which we develop a wide-bandwidth integral AR structure for silicon optics that uses a novel fabrication technique that combines deep reactive ion etching (DRIE) and wafer bonding. We have previously demonstrated a two-layer AR structure for windows over a 1.6:1 bandwidth and are currently fabricating a four-layer coating for a 4:1 bandwidth. Here, we focus on a design for a six-layer structure optimized to give -20 dB reflection between 80 and 420 GHz (5.25:1 bandwidth), which will be useful for future multicolor SZ observations.
", "doi": "10.7907/w1ds-j507", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15208", "collection": "thesis", "collection_id": "15208", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05252023-181654402", "type": "thesis", "title": "Explore the Nature of Dark Matter in the Context of Galaxy Formation", "author": [ { "family_name": "Shen", "given_name": "Xuejian", "orcid": "0000-0002-6196-823X", "clpid": "Shen-Xuejian" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Zurek", "given_name": "Kathryn M.", "orcid": "0000-0002-2629-337X", "clpid": "Zurek-K-M" }, { "family_name": "Golwala", "given_name": "Sunil", "orcid": "0000-0002-1098-7174", "clpid": "Golwala-S-R" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The nature of dark matter (DM) is a fundamental question in modern cosmology. Despite its significant role in various physical processes throughout the Universe, the particle nature of DM remains elusive. With the non-detection of classical candidates (e.g. WIMPs), the theoretical space for DM is becoming increasingly open. This thesis revolves around studying the nature of DM in the context of structure formation and we will focus on a category of DM with self-interactions (SIDM), which can be constrained only through astrophysical probes if DM has no coupling with the standard model particles. Utilizing advanced cosmological hydrodynamical simulations, we examine the effects of DM elastic and dissipative self-interactions on galaxy structure and their interplay with baryonic physics processes. Our numerical studies encompass a range of systems, such as Local dwarf galaxies, massive galaxy clusters in the Local Universe, and rare massive quasar-host galaxies at high redshift (z \u2273 6). In Local dwarf galaxies, we analyze the unique signatures of dissipative self-interacting DM (dSIDM) with typical self-interaction cross-section \u03c3/m ~ 0.1-10 cm\u00b2 g\u207b\u00b9 and dissipation factor ~ 0.5. We find a universal cuspy central density profile and systematic changes in halo morphology in dSIDM. By comparing our results with observations, we derive constraints for effective parameters of dSIDM and identify the parameter space where it remains viable and exhibits interesting observational implications. For a similar type of dSIDM with fairly low \u03c3/m \u227e 0.05 cm\u00b2 g\u207b\u00b9, we also explore the possibility that the direct collapse of dSIDM halos at high redshift can seed supermassive black holes and serve as progenitors for massive bright quasars observed at high redshift. This scenario predicts a large population of quiescent supermassive black holes (SMBHs) at high redshift, which could be tested by future LISA observations. Lastly, in Local massive galaxy clusters, we compare the X-ray morphology of hot gas in observed clusters with simulations of elastic SIDM. Although SIDM models with large interaction cross-sections (\u03c3/m \u2273 0.5 cm\u00b2 g\u207b\u00b9) are favored, uncertainties from cooling and feedback physics in galaxy clusters must be taken into account. This thesis summarizes the findings and constraints on DM properties, with a particular emphasis on its potential self-interactions, as derived from a combination of research projects.", "doi": "10.7907/zccb-sr51", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15169", "collection": "thesis", "collection_id": "15169", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05142023-053312973", "type": "thesis", "title": "Constraints on the Polarized Dust and the Cosmic Microwave Background Using BICEP / Keck Array Series of Telescopes", "author": [ { "family_name": "Kefeli", "given_name": "Sinan", "orcid": "0000-0002-8604-3516", "clpid": "Kefeli-Sinan" } ], "thesis_advisor": [ { "family_name": "Bock", "given_name": "James J.", "orcid": "0000-0002-5710-5212", "clpid": "Bock-J-J" } ], "thesis_committee": [ { "family_name": "Golwala", "given_name": "Sunil", "orcid": "0000-0002-1098-7174", "clpid": "Golwala-S-R" }, { "family_name": "Bock", "given_name": "James J.", "orcid": "0000-0002-5710-5212", "clpid": "Bock-J-J" }, { "family_name": "Dor\u00e9", "given_name": "Olivier P.", "orcid": "0000-0001-7432-2932", "clpid": "Dor\u00e9-O" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "BK18 data consists of all the data taken by the Bicep2, Keck Array, and Bicep3 CMB polarization experiments, as well as publicly available WMAP and Planck maps. The Q/U maps reach depths of 2.8, 2.8 and 8.8 \u00b5KCMB arcmin at 95, 150, and 220 GHz respectively over an effective area of ~ 600 square degrees at 95 GHz and ~ 400 square degrees at 150 and 220 GHz. The likelihood analysis yields r < 0.036 at 95% confidence, with unbiased simulations yielding \u03c3(r)=0.009. The multi-component model that is used in the likelihood analysis consists of lensed-\u039bCDM, tensor modes, and polarize dust and synchrotron components. Foreground model consists of thirteen parameters, some of which are estimated in the likelihood analysis with priors derived from larger regions of sky from WMAP and Planck: amplitude, spectral index, and spatial index for dust and sync, as well as their spatial correlation; dust frequency decorrelation and tensor-to-scalar-ratio. Spectral index for dust emission no longer requires a prior taken from measurements on other regions of the sky.
\r\n\r\nIn the BK papers, EE spectra are not used to derive the model, however the spectra agree well with the assumption that EE/BB = 2 for dust. In this thesis we expand on this assumption, sharing the results for the EE/BB estimate for dust when this is a free parameter in the likelihood calculation. We use the map-based and spectral-based band difference approaches to include E-modes in the likelihood analysis. In the BK papers, dust parameters are assumed to be constant over the sky. We will go over the likelihood work on the spatial constraints for the dust spectral index to examine the validity of this assumption.
", "doi": "10.7907/7a55-p606", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:16090", "collection": "thesis", "collection_id": "16090", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072023-051240339", "primary_object_url": { "basename": "Thesis-Yanlong_Shi.pdf", "content": "final", "filesize": 11412219, "license": "other", "mime_type": "application/pdf", "url": "/16090/2/Thesis-Yanlong_Shi.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Assembly of Intermediate-Mass Black Holes Along Star Formation", "author": [ { "family_name": "Shi", "given_name": "Yanlong", "orcid": "0000-0002-0087-3237", "clpid": "Shi-Yanlong" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Dor\u00e9", "given_name": "Olivier P.", "orcid": "0000-0001-7432-2932", "clpid": "Dor\u00e9-O" } ], "local_group": [ { "literal": "TAPIR" }, { "literal": "div_pma" } ], "abstract": "Intermediate-mass black holes (IMBHs) are poorly observed and not as well understood as stellar-mass black holes (BHs) and supermassive black holes (SMBHs). However, they can be important to complement the formation scenario of massive BHs other than stellar-mass ones and reconcile the existence of some high-energy sources in the Universe. The thesis studies the assembly of IMBHs in star-forming giant molecular clouds (GMCs) of \u223c 5 \u2013 500 pc and \u223c 10\u2074 \u2013 10\u00b9\u2070 M_\u2299, which are realistic environments for some scenarios of IMBH formation, including runaway collisions in dense star clusters and super-Eddington accretion onto \u223c 100 M_\u2299 BH seeds like remnants of massive stars.
\r\n\r\nWe first inspect the runaway-collision scenario where IMBHs form as remnants of \u201cquasi-stars\u201d after stellar collisions. Density profiles of young massive clusters can be important for this scenario but are missing observational hints. We measure density profiles of cluster populations in star-formation simulations in GMCs and conduct both analytic derivations and Monte-Carlo simulations to estimate the mass of the quasi-star in different clusters. The analytic expression is in approximate agreement with observations.
\r\n\r\nThe following three chapters are about the super-Eddington accretion scenario. The first question to solve is the availability of super-Eddington accretion in turbulent and star-forming environments. We run simulations of BH accretion in GMCs with star formation based on FIRE-2 physics. We find that dense clumps generated by stellar feedback and turbulence can feed BHs at high accretion rates. We also conclude that GMCs with high surface densities are favored for super- or hyper-Eddington accretion, in which self-gravity dominates over stellar feedback.
\r\n\r\nAfter convincing the availability of super-Eddington accretion in dense GMCs, we study the self-regulation of BH accretion through its feedback. We construct a sub-grid model of BH accretion and feedback, including radiation, winds/jets, and relativistic diffusive cosmic rays. We find that super-Eddington accretion is still achievable with proper radiative feedback models but is challenged by BH mechanical feedback. We also quantify BH feedback effects and find that they can be analytically explained with momentum-driven arguments. Moreover, we study the effects of multiple sub-grid parameters and BH feedback\u2019s impact on star formation in GMCs.
\r\n\r\nFinally, we study another mode of accretion due to steady gas inflow towards BHs. This complements the missing interaction between stars and BHs in previous studies. Along with star formation, star clusters form and merge hierarchically, creating deep potential wells to capture BHs. At the late stage of the simulation, a \u223c 10 pc disk structures form. The gas inflow rate can be \u223c 10 M_\u2299/yr. We find a non-trivial strong toroidal magnetic field in the disk, which is thermally heated and ionized by feedback from stars.
", "doi": "10.7907/thmn-7h07", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14988", "collection": "thesis", "collection_id": "14988", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07262022-180810750", "primary_object_url": { "basename": "pike_sean_2023.pdf", "content": "final", "filesize": 15883924, "license": "other", "mime_type": "application/pdf", "url": "/14988/1/pike_sean_2023.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Exploring the Mass Accretion Rates of Neutron Star X-Ray Binaries and the Properties of Cadmium Zinc Telluride for Hard X-Ray Astronomy", "author": [ { "family_name": "Pike", "given_name": "Sean Nicholas", "orcid": "0000-0002-8403-0041", "clpid": "Pike-Sean-Nicholas" } ], "thesis_advisor": [ { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" } ], "thesis_committee": [ { "family_name": "Weinstein", "given_name": "Alan Jay", "orcid": "0000-0002-0928-6784", "clpid": "Weinstein-Alan-J-Physics" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "My thesis work has consisted of investigations of several X-ray binaries using the Nuclear Spectroscopic Telescope Array (NuSTAR), as well as characterization of NuSTAR-like hybrid X-ray detectors for the purpose of future hard X-ray obser- vatories. I present analyses of three X-ray binaries in particular, probing different luminosities with the goal of investigating accreting neutron stars at different ac- cretion rates. At high accretion rates, approaching and moderately exceeding the Eddington limit, I performed a timing and spectral analysis of the neutron star su- pergiant X-ray binary SMC X-1. I tracked the transient pulsations of this source at different apparent luminiosities and analyzed the source spectrum as pulsations evolved in order to infer that the mechanism leading to pulsation transience is likely related to periodic obscuration by a warped accretion disk. At lower accretion rates, I studied the low-mass X-ray binary GRS 1741.9\u20132853. I proposed for NuSTAR observations of this source in outburst, and as a result I observed two type-I X- ray bursts originating from the source. I performed a time-resolved spectroscopic study of each of these bursts, and I used the results of this analysis to determine the composition of the burning material and the distance to the source from Earth. The third X-ray binary I present is MAXI J1848\u2013015. This source was discovered by the Monitor of All-sky X-ray Image (MAXI) in winter 2020 at which point its luminosity was abnormally low for an accreting compact object in outburst, making it a good candidate for the study of neutron star X-ray binaries at low accretion rates. I performed rapid follow-up of the source with NuSTAR, and I performed a detailed spectroscopic study of the NuSTAR observations, utilizing relativistic disk reflec- tion models in order to determine that the source is in fact a black hole rather than a neutron star. Complementing these observational astrophysics projects, I character- ized Cadmium Zinc Telluride (CZT) detectors procured from Redlen Technologies in order to determine their usefulness for future hard X-ray observatories. Using Python, I produced an adaptable calibration and analysis pipeline with which to analyze noise, leakage current, spectral, and other data which I obtained in the lab. Using this pipeline, I determined that the material available from Redlen shows good uniformity, yield, and superior spectral resolution up to high photon energies, making it an excellent candidate for application to hard X-ray astronomy.
", "doi": "10.7907/ctjf-2768", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14447", "collection": "thesis", "collection_id": "14447", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12142021-081118617", "type": "thesis", "title": "Probing the Corona in Active Galactic Nuclei Using Broadband X-Ray Spectroscopy", "author": [ { "family_name": "Kamraj", "given_name": "Nikita", "orcid": "0000-0002-3233-2451", "clpid": "Kamraj-Nikita" } ], "thesis_advisor": [ { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" } ], "thesis_committee": [ { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Active Galactic Nuclei (AGN) are some of the most luminous sources in our known universe, powered by large-scale accretion onto a supermassive black hole. Supermassive black holes are known to co-evolve with their host galaxies, with AGN playing an important role in regulating feedback in galaxies by depositing large amounts of energy through outflows and jets. While studies of AGN have dated back to the 1960's when the first quasar was discovered, much is still unknown about the key source powering the X-ray emission in AGN, i.e. the corona. In my thesis, I present detailed investigations of the properties of AGN coronae using broadband X-ray spectroscopic techniques. This work utilizes spectroscopic data taken with the NuSTAR telescope, which has revolutionized studies of the corona. Being the first focusing high energy X-ray telescope in orbit, NuSTAR's high sensitivity at hard X-ray energies (> 10 keV) has enabled robust measurements of fundamental properties of the corona, such as its temperature, from single epoch observations of AGN for the first time. In the first study presented in this thesis, I performed measurements of coronal temperature in a sample of 46 NuSTAR-observed AGN through fitting X-ray spectral models for each object. My analysis showed the temperature of the corona to be regulated by electron-positron pair production and annihilation processes. From this sample, I identified an AGN with an unusually low coronal temperature, 2MASX J19301380+3410495. I modeled the broadband X-ray spectrum of this object in detail using multi-epoch X-ray observations taken with the Swift, XMM-Newton, and NuSTAR telescopes, and found the object to also belong to a rare class of X-ray obscured but optically unobscured AGN. Using multi-wavelength information, I elucidated the nature of the complex obscuration present in 2MASX J19301380+3410495. In recent work presented in this thesis, I compiled one of the largest samples of unobscured AGN with high quality NuSTAR X-ray spectra in order to characterize how the physical properties of the corona relate to fundamental accretion parameters in AGN, such as the Eddington ratio and mass of the supermassive black hole. Finally, I discuss possible future work directed at investigating other enigmatic AGN similar to 2MASX J19301380+3410495 that have conflicting optical and X-ray classifications. Using techniques such as multi-wavelength spectropolarimetry in addition to X-ray spectroscopy, it may be possible to unveil the mechanisms of obscuration within these exotic sources that challenge classical pictures of AGN structure.
", "doi": "10.7907/cag4-k322", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14299", "collection": "thesis", "collection_id": "14299", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07122021-033018615", "primary_object_url": { "basename": "Thesis_Cheng_Yun_Ting.pdf", "content": "final", "filesize": 6404023, "license": "other", "mime_type": "application/pdf", "url": "/14299/1/Thesis_Cheng_Yun_Ting.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Cosmology and Astrophysics with Intensity Mapping", "author": [ { "family_name": "Cheng", "given_name": "Yun-Ting", "orcid": "0000-0002-5437-0504", "clpid": "Cheng-Yun-Ting" } ], "thesis_advisor": [ { "family_name": "Bock", "given_name": "James J.", "orcid": "0000-0002-5710-5212", "clpid": "Bock-J-J" } ], "thesis_committee": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Dor\u00e9", "given_name": "Olivier P.", "orcid": "0000-0001-7432-2932", "clpid": "Dor\u00e9-O" }, { "family_name": "Chang", "given_name": "Tzu-Ching", "orcid": "0000-0001-5929-4187", "clpid": "Chang-Tzu-Ching" }, { "family_name": "Bock", "given_name": "James J.", "orcid": "0000-0002-5710-5212", "clpid": "Bock-J-J" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Intensity mapping (IM) has emerged as a promising technique to probe the largescale structures and galaxy formation and evolution across cosmic history. As IM measures the aggregate emission from all sources, it can overcome the limitation of conventional detection-based observations, where the emission from diffuse populations and high-redshift faint galaxies cannot be resolved individually. As several IM experiments will come online in the next decade, demand for IM modeling and data analysis strategies has increased. In this thesis, we present a range of analysis techniques, theoretical modeling, and data analysis results related to IM.
\r\n\r\nIn Chapter 2, we aim to answer the question: When should we use IM? We present a formalism to describe both IM and galaxy detection (GD) approaches, and use it to quantify their individual performance when measuring the large-scale structure (LSS). With this formalism, we can identify the scenarios where each approach is advantageous. We also develop a simple metric for determining the optimal strategy to map the LSS with future experiments.
\r\n\r\nIn Chapters 3 and 4, we interrogate methods for improving the line intensity mapping (LIM) analysis. LIM traces the three-dimensional structure of the universe by probing the emission field from a spectral line. One particular challenge for LIM is to separate the target line signals from interloper lines along the line of sight in order to extract the desired cosmological and astrophysical information. Previously proposed methods of line de-blending, such as masking and cross-correlation, rely on the external galaxy tracers, but sometimes a galaxy catalog with sufficient depth and sky coverage does not exist. Therefore, we develop two new methods for performing line de-confusion that do not require any external information. The first method (Chapter 3) uses the distinct shape of large-scale two-dimensional power spectra of signals and interlopers to distinguish the line emission from different redshifts. The second method (Chapter 4) reconstructs the intensity maps of individual lines from LIM data in the phase space, using multiple lines from the same source to identify the source redshift. We show that both of our methods are able to effectively extract desired line signals from the upcoming LIM experiments.
\r\n\r\nIn Chapter 5, we discuss the application of IM for studying the extragalactic background light (EBL), the integrated light from all sources of emission in the universe. Previous studies on the fluctuations of the EBL indicate that the intra-halo light (IHL) has a significant contribution to the near-infrared EBL. Chapter 5 presents the results on probing the IHL using a stacking analysis of images from the Cosmic Infrared Background Experiment (CIBER). CIBER is a rocket-borne experiment designed to image and perform photometry of the near-infrared EBL. Our results suggest that at z \u223c 0.3 the IHL comprises a large fraction of light associated with \u223c L\u2217 galaxies, implying that the IHL accounts for a non-negligible fraction of the near-infrared cosmic radiation budget.
\r\n\r\nIn Chapter 6, we present a forecast on the EBL constraints with the upcoming SPHEREx mission. We consider cross correlating SPHEREx intensity maps with galaxy catalogs from several current and future surveys. Our model predicts that the EBL spectrum as a function of redshift can be detected from the local universe to the epoch of reionization.
\r\n\r\nThe analysis techniques developed in this thesis can help better extract the information from the IM data; the future IM experiments will extend our current works on investigating the EBL. Therefore, the research in this thesis provides important toolkits and foundations for upcoming IM experiments.
", "doi": "10.7907/p41n-8698", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14332", "collection": "thesis", "collection_id": "14332", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08232021-201252167", "primary_object_url": { "basename": "Yuguang_Chen_Phd_final.pdf", "content": "final", "filesize": 14048409, "license": "other", "mime_type": "application/pdf", "url": "/14332/2/Yuguang_Chen_Phd_final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Absorption and Emission of Neutral Hydrogen around High-Redshift Star-Forming Galaxies", "author": [ { "family_name": "Chen", "given_name": "Yuguang", "orcid": "0000-0003-4520-5395", "clpid": "Chen-Yuguang" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "thesis_committee": [ { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The circumgalactic medium (CGM) mediates the interaction between galaxies and their surrounding gas, and ultimately controls the evolutionary trajectory of its host galaxies. Among the CGM gas, neutral hydrogen (H I) constitutes an important component observationally. This thesis focuses on characterizing the circumgalactic H I around z = 2-3 star-forming galaxies using absorption and emission techniques in the Keck Baryonic Structure Survey (KBSS) fields.
\r\n\r\nChapter 2 provides a new statistical measurement of the distribution and kinematics of H I in the CGM using Lyman-alpha (Ly\u03b1) absorption signatures. We constructed the KBSS Galaxy Pair Sample (KGPS), which contains > 200,000 distinctive foreground-background galaxy pairs with projected distances of 0.02-4 Mpc. We compared the observed H I absorption map with a cosmological zoom-in simulation, and found qualitative agreement. Furthermore, using an analytic model, we show that the observed H I distribution can be explained by a combination of outflow, accretion, and Hubble expansion.
\r\n\r\nChapters 3 and 4 explore the Ly\u03b1 emission, one of the strongest spectral features of H I, from the spatially resolved spectroscopy, by using the new high-sensitivity integral field unit, Keck Cosmic Web Imager (KCWI). We introduce the use of \"cylindrically projected 2D (CP2D) spectra,\" which map the average Ly\u03b1 spectral profile over a specified range of azimuthal angle, as a function of impact parameter, around galaxies. We find that the averaged CP2D spectrum is most consistent with the expected signatures of Ly\u03b1 resonant scattering through outflowing gas.
\r\n\r\nFurthermore, Chapter 3 investigates the statistical relation between Ly\u03b1 emission and its orientation relative to the galaxy major axis for 59 galaxies that have been observed with both KCWI and high-spatial-resolution imaging instruments. We find that the extended Ly\u03b1 emission along the galaxy projected major and minor axes are statistically indistinguishable at projected distance \u227e 30 physical kpc, indicating that resonant scattering of Ly\u03b1 by outflowing H I occurs in a statistically uniform halo, independent of galaxy orientation.
\r\n\r\nChapter 4 provides ongoing research on the connection between Ly\u03b1 emission in the CGM and the host galaxy properties. Our preliminary result from 110 star-forming galaxies at z \u2243 2-3 demonstrates that the host galaxy affects the production and escape of Ly\u03b1 emission at \u227e 15 kpc as expected. However, the faint envelopes of Ly\u03b1 emission reach to similar spatial distances (\u2273 30 kpc) regardless of their host galaxies.
\r\n\r\nFinally, we document the standard data reduction procedure and data products of the KBSS-KCWI survey in Appendix A.
", "doi": "10.7907/gsr9-9y83", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14581", "collection": "thesis", "collection_id": "14581", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05162022-191523325", "primary_object_url": { "basename": "main.pdf", "content": "final", "filesize": 6635412, "license": "cc_by_nc_sa", "mime_type": "application/pdf", "url": "/14581/1/main.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Dwarf Galaxies in the Local Universe as Probes of Stellar and Galactic Evolution", "author": [ { "family_name": "de los Reyes", "given_name": "Mithi Alexa Caballes (Mia)", "orcid": "0000-0002-4739-046X", "clpid": "de-los-Reyes-Mithi-Alexa-Caballes" } ], "thesis_advisor": [ { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "thesis_committee": [ { "family_name": "Kasliwal", "given_name": "Mansi M.", "orcid": "0000-0002-5619-4938", "clpid": "Kasliwal-Mansi-M" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Ravi", "given_name": "Vikram", "orcid": "0000-0002-7252-5485", "clpid": "Ravi-Vikram" } ], "local_group": [ { "literal": "3MT Competition (Caltech)" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Low-mass \"dwarf\" galaxies are numerous, diverse, and relatively simple, making them excellent laboratories for understanding both stellar and galactic evolution. Dwarf galaxies (galaxies with stellar masses 107-109 solar masses) in the nearby universe (out to distances of ~10 Mpc from our own Milky Way, or redshifts z \u227e 0.01), are a particularly interesting population due to their proximity, which allows us to study them in detail on both spatially-resolved and global scales. In this thesis, I present a variety of observational studies investigating star formation and chemical enrichment within nearby dwarf galaxies.
\r\n\r\nI first use the chemical abundances of individual stars within Local Group dwarf spheroidal galaxies (dSphs) to understand how past stars lived and died. I use this \"galactic archaeology\" approach to test theoretical models of Type Ia supernovae by investigating the nucleosynthetic products of these supernovae. For example, using medium-resolution spectra from DEIMOS, I measure manganese abundances that place observational constraints on the masses of Type Ia progenitors. I also describe a novel method to measure the delay-time distribution of Type Ia SNe in an individual dSph, which probes the number of white dwarfs involved in Type Ia SNe. Finally, I show how galactic archaeology can also be used to trace star formation using a simple chemical evolution model.
\r\n\r\nFor more distant galaxies, in which individual stars cannot be resolved, galaxy properties can be measured on larger scales. Using the integral field spectrograph KCWI, I produce spatially-resolved maps of dwarf galaxies located in extremely under-dense regions called cosmic voids. The dynamical properties of these galaxies provide insight into the formation of dwarf galaxies in the absence of large-scale environmental effects. On even larger spatial scales, I use photometry to measure the global properties of galaxies and understand the physical processes that drive star formation on galaxy-wide scales.
\r\n\r\nThese studies pave the way for future work with ongoing and upcoming surveys that will map out our local universe---and the dwarf galaxies around us---in unprecedented detail.
", "doi": "10.7907/4bz3-2t66", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14199", "collection": "thesis", "collection_id": "14199", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292021-004454950", "primary_object_url": { "basename": "belland_brent_2021.pdf", "content": "final", "filesize": 8199985, "license": "other", "mime_type": "application/pdf", "url": "/14199/1/belland_brent_2021.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Modeling Focal Ratio Degradation, Its Implications for Upcoming Fiber Spectrographs, and the Dynamics of NGC 6822", "author": [ { "family_name": "Belland", "given_name": "Brent Roy", "orcid": "0000-0003-1950-448X", "clpid": "Belland-Brent-Roy" } ], "thesis_advisor": [ { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "thesis_committee": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Howard", "given_name": "Andrew W.", "orcid": "0000-0001-8638-0320", "clpid": "Howard-Andrew-W" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Spectroscopy is a cornerstone of astronomical research, enabling the measurements of abundances and velocities of astronomical objects. Fiber spectroscopy, with its capability to acquire spectra from targets in densely packed fields and position fibers over targets over a large field of view, promises to greatly expand the number of astronomical objects with acquired spectra. Upcoming such projects include the Subaru Prime Focus Spectrograph, which plans to advance cosmology, galactic archaeology, and galactic evolution studies.
\r\n \r\nHowever, with the introduction of fibers to the spectrographic optical path also comes the introduction of the issue of focal ratio degradation (FRD). FRD is the scattering of light traveling through a fiber due to effects such as imperfections in the core-clad interface of a fiber, end face preparation, and stresses imposed on the fibers; thus, FRD is inherent to any fiber-based instrument. Focal ratio degradation scatters light to larger angles, resulting in lost light at the spectrograph and redistribution of light in the light's point spread function at the spectrograph detector. These issues are compounded by the fact that each fiber has a unique FRD 'fingerprint' and stresses during telescope operation can vary and induce dynamically changing FRD. It is also difficult to measure precisely.
\r\n \r\nThis thesis focuses on characterization of FRD. It introduces a novel approach to measuring that permits extraction of the effect of angular misalignment and tests this model on fiber mounted to a Cobra fiber positioner, though this method could be utilized for any optical fiber. The effects of focal ratio degradation on the spectra are simulated and found to affect counts 3-4 pixels from the center of sky lines on the order of 1-2% for changes in FRD at the 3 milliradian level, highlighting the importance of a good understanding of FRD to model sky lines at the level desired for the Subaru prime focus spectrograph.
\r\n \r\nMultiplexed spectroscopy in NGC 6822 from KECK/DEIMOS is also presented. The ancient red giant population in NGC 6822 is prolately rotating, an unusual configuration that has been used as evidence of merger histories in other dwarf galaxies. The distribution of metallicity as a function of radius shows the oldest, most metal-poor stars are the most dispersion dominated. They also reside at larger radii, possibly pointing to a disruptive star formation history that scattered stars from the center of the galaxy over time. Future results from upcoming spectrographic surveys such as the Subaru Prime Focus Spectrograph could illuminate what is driving these peculiar features in NGC 6822 among many other scientific discoveries.
", "doi": "10.7907/rbce-f185", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:13846", "collection": "thesis", "collection_id": "13846", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08042020-182952053", "primary_object_url": { "basename": "Thesis__Mapping_the_Cosmic_Web_Updated.pdf", "content": "final", "filesize": 67326637, "license": "other", "mime_type": "application/pdf", "url": "/13846/1/Thesis__Mapping_the_Cosmic_Web_Updated.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Mapping the Diffuse Universe: Integral Field Spectroscopy of Galaxy Environments", "author": [ { "family_name": "O'Sullivan", "given_name": "D\u00f3nal Brendan", "orcid": "0000-0002-4959-9179", "clpid": "O'Sullivan-D\u00f3nal-Brendan" } ], "thesis_advisor": [ { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" } ], "thesis_committee": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The population of galaxies we see today is the result of billions of years of gas inflows, outflows, mergers, and feedback. To develop any holistic picture of the origin and evolution of galaxies, we thus need to understand their environments. The circumgalactic and intergalactic media (CGM and IGM) - the gas around and between galaxies, respectively - represent a large part of this environment. However, this gas is extremely faint and thus difficult to observe, and only recently have we been able to image it directly. This thesis presents instrumental and observational work focused on revealing galaxy environments in the early universe.
\r\n\r\nChapter 1 presents a brief history of our understanding of galaxies and an overview of our current picture of galaxy formation, including the role played by galaxy environments. In particular, it focuses on presenting the evolution of baryonic structures within a cosmological density field dominated by dark matter.
\r\n\r\nChapter 2 presents instrumental work on the Keck Cosmic Web Imager (KCWI, Morrissey et al. 2018), a new integral field spectrograph (IFS) for the Keck-2 10m telescope designed to study faint, extended emission. As an introduction, I discuss the advantages and disadvantages of integral field spectroscopy for the application of studying galaxy environments, as well as an overview of the prototype instrument - the Palomar Cosmic Web Imager (PCWI, Matuszewski et al. 2010). This chapter focuses primarily on engineering work during the development and testing of KCWI, though I conclude with a brief comparison of PCWI and KCWI performance in measuring the CGM around a high-redshift QSO.
\r\n\r\nChapter 3 presents the development of a software package designed to extract and analyze faint, extended emission in PCWI and KCWI data: CWITools. Although software is often an afterthought in astronomical and observational work, it is likely to become a primary barrier to conducting large IFS surveys of the CGM and IGM. This semi-automated analysis pipeline is presented and released publicly to empower future PCWI and KCWI studies.
\r\n\r\nChapter 4 presents the FLASHES (Fluorescent Lyman-\u03b1 Structures in High-z Environments) pilot survey, published as O'Sullivan et al. 2020. The FLASHES pilot survey is an IFS study of extended HI Lyman-\u03b1 emission in the environments of 48 z = 2.3 - 3.0 QSOs. The FLASHES Survey is the core project of this thesis, enabled by the instrumentation in Chapter 2 and the analysis pipeline developed in Chapter 3. The pilot survey represents the first statistically significant (N \u2273 30) sample of direct CGM observations in its redshift range. As such, it provides the first direct constraints on the 2D morphology, surface brightness profiles, and spatially resolved kinematics of the CGM during this period.
\r\n\r\nChapter 5 presents the first FLASHES follow-up study; deep IFS observations targeting extended Ly\u03b1 1216\u00c5, NV 1240\u00c5, CIV 1549\u00c5, and HeII 1640\u00c5 emission from a subset of FLASHES pilot targets (O'Sullivan et al., in prep). Emission from metals in the CGM is expected to be an order of magnitude or more fainter than its Ly\u03b1, yet is a crucial ingredient in understanding the composition of the gas. Detecting this emission still requires multiple hours on 10m class telescopes. As such, large surveys of the multi-phase CGM remain extremely difficult to conduct. In this chapter, I present detections and upper limits of CGM metal emission around 8 FLASHES targets.
\r\n\r\nChapter 6 presents engineering work on FIREBall-2 (the Faint Intergalactic Redshifted Emission Balloon, second generation), a high-altitude UV telescope and IFS targeting CGM emission in the low-redshift universe (z \u2243 0.7). FIREBall-2 is an ambitious project deploying a novel, electron-multiplying CCD designed to achieve \u2273 50% quantum efficiency in the UV. This technology represents an order of magnitude increase in sensitivity from the microchannel plates used in the GALEX space telescope. FIREBall-2 serves as both an observational project in its own right, studying the low-z CGM, and a pathfinder mission for future UV space missions.
\r\n\r\nFinally, Chapter 7 summarizes the contributions from this thesis and present a brief outlook on a few topics related to observations of galaxy environments.
", "doi": "10.7907/6g0e-n897", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14184", "collection": "thesis", "collection_id": "14184", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272021-232230617", "primary_object_url": { "basename": "Yanjun_Xu_PhD_Thesis_2021.pdf", "content": "final", "filesize": 8993893, "license": "other", "mime_type": "application/pdf", "url": "/14184/1/Yanjun_Xu_PhD_Thesis_2021.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Probing the Inner Accretion Flow Properties Around Black Holes with X-ray Observations", "author": [ { "family_name": "Xu", "given_name": "Yanjun", "orcid": "0000-0003-2443-3698", "clpid": "Xu-Yanjun" } ], "thesis_advisor": [ { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" } ], "thesis_committee": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" }, { "family_name": "Weinstein", "given_name": "Alan Jay", "orcid": "0000-0002-0928-6784", "clpid": "Weinstein-Alan-J-Physics" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" } ], "local_group": [ { "literal": "Space Radiation Laboratory" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Accretion, governed by gravity, is a fundamental source of energy in the universe. Accretion is important to the growth and evolution of black holes, as well as the structure in the universe on larger scales. Accretion disks around supermassive black holes are likely to have produced most of the ionizing radiation in the universe since the epoch of reionization. Outflows launched from the accretion disk of black holes, either in the form of disk winds or jets, provide a feedback mechanism that plays an important role in the co-evolution of black holes and their host galaxies. It has been decades since the fundamental theories about black hole accretion were established. Black hole accretion has been widely studied in active galactic nuclei (AGNs) and black hole binaries ever since the early days of X-ray astronomy. However, important questions still remain regarding the fundamental physical properties of black holes, the structure and geometry of accretion disks and coronae, and the nature of disk winds and jets.
\r\n\r\nIn this thesis, I report results from recent X-ray observations of an ultra-luminous infrared galaxy with a central AGN, IRAS 05189\u20132524, and several black hole X-ray binaries (or black hole candidates for those currently lacking dynamical mass determinations), IGR J17091\u20133624, MAXI J1535\u2013571, Swift J1658.2\u20134242, MAXI J1631\u2013479, and MAXI J1820+070. Most of the black hole X-ray binaries studied in this thesis were uncatalogued sources and were discovered as bright Galactic X-ray transients over the past few years, offering great opportunities for investigating black hole accretion with high quality datasets. The launch of the NuSTAR telescope in 2012 has brought the advanced capabilities of performing high sensitivity observations in the hard X-ray band and remaining free from pile-up for bright Galactic sources, providing new angles for the detailed study of observational phenomena around both supermassive and stellar-mass accreting black holes. I study the inner accretion flow properties around black holes in the above objects by conducting spectral and timing analyses of the NuSTAR observations and the simultaneous soft X-ray band data from the Swift or XMM-Newton telescope. I have searched for and analyzed various accretion related observational features in these systems (disk reflection spectra, ionized absorption caused by disk winds, quasi periodic oscillations (QPOs) in the X-ray light curves, and unusual aperiodic flux variations or accretion state changes), and interpret the results in terms of the physical properties about the inner accretion flows and the central black holes.
\r\n\r\nBy modeling the relativistic disk reflection spectra, I have found that the inner edge of the optically-thick accretion disk is truncated in IGR J17091\u20133624 during the bright hard state, in MAXI J1631\u2013479 during the very high state, and in MAXI J1820+070 during the faint hard state, whereas the inner accretion disk is consistent with extending down to the ISCO in MAXI J1535\u2013571 and Swift J1658.2\u20134242 during their bright hard states, and in MAXI J1631\u2013479 during its soft state. Based on all the observational evidence gathered in this thesis, the general picture about the accretion flow geometry at different accretion states seems to be more complicated than that from previously well accepted theoretical predictions. In addition, I have measured the black holes spins and inner accretion disk inclinations for IRAS 05189\u20132524, MAXI J1535\u2013571, Swift J1658.2\u20134242, and MAXI J1631\u2013479 using the disk reflection modeling method, where the central black holes are all found to be rapidly spinning. There are also evidence for strong disk winds detected in the X-ray spectra of some of the objects studied in this thesis with interesting implications. I summarize my studies of these individual objects at the end of this thesis in a more broader context of the characteristic behaviors of the population and their general physical implications, and discuss about the possibilities of extending my research in this direction with the upcoming new X-ray missions.
", "doi": "10.7907/9kkg-4b58", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:13559", "collection": "thesis", "collection_id": "13559", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11082019-215827632", "type": "thesis", "title": "Signatures of the r-Process in Ancient Stellar Populations Using Barium Abundances", "author": [ { "family_name": "Duggan", "given_name": "Gina Elizabeth", "orcid": "0000-0002-9256-6735", "clpid": "Duggan-Gina-Elizabeth" } ], "thesis_advisor": [ { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "thesis_committee": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "orcid": "0000-0001-5390-8563", "clpid": "Kulkarni-S-R" }, { "family_name": "Cohen", "given_name": "Judith G.", "orcid": "0000-0002-8039-4673", "clpid": "Cohen-J-G" }, { "family_name": "Guhathakurta", "given_name": "Puragra", "orcid": "0000-0001-8867-4234", "clpid": "Guhathakurta-Puragra" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "For over sixty years scientists have known that a large percentage of heavy elements are created by the rapid neutron-capture process (r-process). However, a clear picture of where the r-process occurs has remained elusive. Many astrophysical origins have been proposed -- each with a range of possible chemical yields and rates. Discovering which origin (or combinations of origins) truly produce the heavy elements we see on Earth is a daunting task.
\r\n\r\nThis thesis seeks to provide observational constraints to pinpoint the dominant origin of the r-process. The majority of this thesis uses Galactic Archaeology to look for r-process signatures in ancient stellar populations (e.g., dwarf galaxies and globular clusters). These ancient stellar populations provide the clearest \"experiments\" to observe how quickly and how much r-process was created. The r-process signature we observe is the amount of barium in individual red giant branch stars in these stellar populations. Chapter 2 discusses how these barium measurements are made from individual extragalactic stars and presents the largest catalog of barium abundances in dwarf galaxies to date.
\r\n\r\nChapter 3 compares the r-process signature -- barium -- to other elements (e.g., magnesium and iron) in the same galaxy to see how the timescale of r-process enrichment compares to the other abundances (whose origins are known). This analysis found that the r-process timescale was more delayed than core-collapse supernovae. This points to neutron star mergers (NSMs) as the dominant source of r-process in the early history of dwarf galaxies.
\r\n\r\nChapter 4 uses a galactic chemical evolution model to test what r-process timescales, yields, and rates are needed to recreate the observations presented in Chapter 2. Preliminary results indicate that NSMs must be included in order for the model to match the observations. In addition, Chapter 4 presents what yield of barium is needed from NSMs to recreate the observations.
\r\n\r\nChapter 5 tests if the stars in the globular cluster M15 were enriched by the r-process after they were born. M15 has an unusual abundance pattern with ~ 1 dex variation in r-process abundances even though most other elements, including iron, do not show a variation. New measurements of barium abundances in main sequence and red giant branch stars of M15 show that the stars were born with their r-process enrichment. This means that an r-process event occurred quickly after the cluster was born -- while it was still forming stars -- and resulted in uneven enrichment.
\r\n\r\nFinally, Chapter 6 presents a solution to one of the technical challenges in locating the sites of r-process nucleosynthesis. Chapter 6 describes how to accurately measure the position and orientation of the CCDs in Zwicky Transient Facility's (ZTF's) camera. ZTF is a transient survey that -- among other science goals -- searches for the electromagnetic counterpart of NSM detections with LIGO. The work included in this chapter increased the survey efficiency of ZTF, which will aid ZTF in localizing transient events, including NSMs. Following up NSMs found by LIGO can provide direct measurements of the amount of r-process material created by NSMs.
\r\n\r\nAltogether, this thesis has made strides to identifying the origin of the r-process. Chapters 3 and 4 identify NSMs as the dominant source of r-process elements in dwarf galaxies. However, Chapter 5 found that globular cluster M15 needs a r-process event to occur quickly -- quicker than is typically expected from a NSM. The observational constraints that have resulted from this thesis provide important clues to where the heaviest elements are made.
", "doi": "10.7907/D0A8-BM50", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13783", "collection": "thesis", "collection_id": "13783", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06052020-225431864", "type": "thesis", "title": "Leveraging the Rest-Ultraviolet and Rest-Optical Spectra of Galaxies at 2 < z < 3", "author": [ { "family_name": "Theios", "given_name": "Rachel Lauren", "orcid": "0000-0002-4236-1037", "clpid": "Theios-Rachel-Lauren" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "thesis_committee": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Galaxies at the peak of cosmic star formation (2 < z < 3) are fundamentally different from local galaxies, in terms of the properties of their massive stellar populations and physical conditions in the interstellar medium (ISM). This thesis presents a detailed analysis of the stellar and nebular properties of high-redshift galaxies, using the rest-frame UV and rest-frame optical spectra of galaxies from the Keck Baryonic Structure Survey (KBSS), a large, targeted spectroscopic survey of galaxies at 2 < z < 3.
\r\n\r\nChapter 2 compares inferences of dust attenuation, star formation, and metallicity from strong nebular emission lines, the far-UV continuum, and spectral energy distribution (SED) fits. These results indicate that the majority of high-redshift galaxies display different dust properties than those at low redshift, and that the assumption of a dust attenuation curve can dramatically change inferred properties such as star formation rates (SFRs). I find that SFRs estimated using different methods only agree under specific combinations of assumptions, and caution that SFR calibrations established in the local Universe do not apply at higher redshifts.
\r\n\r\nChapter 3 utilizes rest-UV absorption lines to study the outflow kinematics of high-redshift galaxies. I compare several velocity metrics used in the literature, and search for correlations between outflow velocity and galaxy properties. These results are consistent with the picture of winds driven by momentum injected into the ISM by stellar feedback. I confirm that large-scale outflows are ubiquitous at high redshift due to these galaxies' high SFRs and compact sizes.
\r\n\r\nFinally, Chapter 4 analyzes the systematic uncertainties involved in fitting stellar population synthesis (SPS) models to rest-UV spectra as well as the full SEDs of galaxies. I quantify differences in galaxy parameters estimated using different combinations of models and assumptions, and explore the dependence of the rest-UV portion of model spectra on stellar metallicity and population age.
", "doi": "10.7907/kj2p-j718", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13683", "collection": "thesis", "collection_id": "13683", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04272020-121646677", "primary_object_url": { "basename": "thesis-jhunacek-20200427b.pdf", "content": "final", "filesize": 45490715, "license": "other", "mime_type": "application/pdf", "url": "/13683/1/thesis-jhunacek-20200427b.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "TIME: A Millimeter-Wavelength Grating Spectrometer Array for [CII] / CO Intensity Mapping", "author": [ { "family_name": "Hunacek", "given_name": "Jonathon Robert", "orcid": "0000-0001-7066-226X", "clpid": "Hunacek-Jonathon-Robert" } ], "thesis_advisor": [ { "family_name": "Bock", "given_name": "James J.", "orcid": "0000-0002-5710-5212", "clpid": "Bock-J-J" } ], "thesis_committee": [ { "family_name": "Zmuidzinas", "given_name": "Jonas", "orcid": "0000-0002-3330-5439", "clpid": "Zmuidzinas-J" }, { "family_name": "Golwala", "given_name": "Sunil", "orcid": "0000-0002-1098-7174", "clpid": "Golwala-S-R" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Bock", "given_name": "James J.", "orcid": "0000-0002-5710-5212", "clpid": "Bock-J-J" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "In this thesis I review the design, fabrication, and initial engineering deployment of the TIME (Tomographic Ionized-carbon Mapping Experiment) instrument. TIME seeks to make a first detection of the clustering amplitude of the power spectrum of redshifted [CII] emission from the Epoch of Reionization (z = 5-9). [CII], the 157.7 \u00b5m fine-structure line of singly ionized carbon, traces star formation on large scales, providing a new method for constraining the contribution of star formation to the Reionization process. [CII] intensity mapping complements traditional galaxy surveys by using spatially-broad beams to integrate signal from the many faint sources thought to be responsible for the bulk of the integrated emission from galaxies. TIME covers the 200-300 GHz atmospheric window, which also enables the study of lower-redshift CO emission (z = 0.5-2), a tracer of molecular gas in the period following the peak of cosmic star formation. The full TIME instrument consists of 32 single-polarization grating spectrometers with a resolution R ~ 100. Each spectrometer consists of an input feedhorn coupled to parallel plate waveguide with a curved diffraction grating, which focuses the diffracted light onto an output arc populated by 60 transition-edge sensor (TES) bolometers at 250 mK. The 1920 total detectors couple to the output of the parallel plate waveguide with a direct-absorbing micro-mesh and are organized into buttable arrays covering 4 spatial by either 12 (HF) or 8 (LF) spectral pixels. A partial TIME instrument was field tested in early 2019 on the ARO APA 12m dish at Kitt Peak. We intend to return to Kitt Peak in late 2020 to begin initial science observations.
\r\n", "doi": "10.7907/hp2n-8495", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13736", "collection": "thesis", "collection_id": "13736", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292020-133139370", "primary_object_url": { "basename": "Escala_Ivanna_2020.pdf", "content": "final", "filesize": 5858207, "license": "other", "mime_type": "application/pdf", "url": "/13736/2/Escala_Ivanna_2020.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Elemental Abundances in the Local Group: Tracing the Formation History of the Great Andromeda Galaxy", "author": [ { "family_name": "Escala", "given_name": "Ivanna Ashley", "orcid": "0000-0002-9933-9551", "clpid": "Escala-Ivanna-Ashley" } ], "thesis_advisor": [ { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "thesis_committee": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" }, { "family_name": "Guhathakurta", "given_name": "Puragra", "orcid": "0000-0001-8867-4234", "clpid": "Guhathakurta-Puragra" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The Local Group (LG) is an environment accessible to detailed studies of galaxy formation, providing a complement to the early universe. In particular, spectroscopy of resolved stellar populations in the LG provides kinematical and chemical information for individual stars that can be used to infer the history of L\u2605 galaxies like the Milky Way (MW) and Andromeda (M31).
\r\n\r\nThe Gaia revolution in the MW, combined with spectroscopy from APOGEE and other surveys, has enabled comprehensive observational studies of the MW's formation history. In addition, comparisons to simulations can be leveraged to maximally utilize such observational data to probe the hierarchical assembly of galaxies. Toward this goal, I have analyzed simulations of chemical evolution in LG dwarf galaxies to assess their ability to match observations.
\r\n\r\nThe exquisite detail in which the MW has been studied is currently not achievable in any other L\u2605 galaxy. For this reason, the MW is a template for our understanding of galaxy formation. M31 is the only external galaxy that we can currently hope study in a level of detail approaching the MW. Studies of M31 have recently taken on greater significance, given the growing body of evidence that its formation history differs substantially from that of the MW.
\r\n\r\nIn an era of limited information about elemental abundances in M31, I have developed a technique to apply spectral synthesis to low-resolution stellar spectroscopy in order to measure abundances for individual giant stars in distant LG galaxies. Through undertaking the largest deep, spectroscopic survey of M31 to date with my collaborators, this has resulted in the first measurements of the elemental abundances in the inner stellar halo and stellar disk of M31, and the largest homogeneous catalog of elemental abundances in M31. With this foundational work, we have opened the doors to detailed studies of the chemical composition of M31.
\r\n\r\nNow, we can begin to ask--and answer--what differences in the elemental abundances of the M31 and the MW imply for our knowledge of galaxy formation in the broader universe. At the cusp of next-generation observational facilities and theoretical simulations, we can only advance toward this goal.
", "doi": "10.7907/16bk-vm98", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:11369", "collection": "thesis", "collection_id": "11369", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01312019-123521450", "type": "thesis", "title": "Core-Collapse Supernova Physics in the Multi-Messenger Era", "author": [ { "family_name": "Gossan", "given_name": "Sarah Elizabeth", "orcid": "0000-0002-8138-9198", "clpid": "Gossan-Sarah-Elizabeth" } ], "thesis_advisor": [ { "family_name": "Weinstein", "given_name": "Alan Jay", "orcid": "0000-0002-0928-6784", "clpid": "Weinstein-Alan-J-Physics" } ], "thesis_committee": [ { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" }, { "family_name": "Adhikari", "given_name": "Rana", "orcid": "0000-0002-5731-5076", "clpid": "Adhikari-R" }, { "family_name": "Chen", "given_name": "Yanbei", "orcid": "0000-0002-9730-9463", "clpid": "Chen-Yanbei" }, { "family_name": "Weinstein", "given_name": "Alan Jay", "orcid": "0000-0002-0928-6784", "clpid": "Weinstein-Alan-J-Physics" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "LIGO" }, { "literal": "div_pma" } ], "abstract": "Eighty-five years following the historic proposal that core-collapse supernovae accompanied the transition of evolved massive stars to neutron stars [1], the mechanism through which these collapsing stars explode remains uncertain. While supernovae are observed on a daily basis across the electromagnetic spectrum, neutrinos and gravitational waves, emitted from the very heart of the core-collapse supernova central engine, provide a direct glimpse of the dynamics driving the explosion. The joint gravitational wave and electromagnetic observations of a colliding neutron star binary system on 17th August 2017 heralded a new era for multi-messenger astronomy [2]. The next galactic core-collapse supernova presents an unparalleled opportunity to directly probe core-collapse supernova physics and the explosion mechanism.
\r\n\r\nThis thesis explores a number of topics in multi-messenger astronomy and core-collapse supernova physics. First, it tackles the observation problem; detailing an astrophysically motivated search protocol for gravitational waves from core-collapse supernovae triggered by observations of neutrino and/or electromagnetic counterparts. Applying these methods to a number of hypothetical observational scenarios, it presents sensitivity estimates for the second generation of gravitational wave interferometric detectors to both realistic and speculative emission mechanisms associated with core-collapse supernovae. Next, it addresses the prospects for post-detection inference; developing a Bayesian toolkit to interpret gravitational wave observations from core-collapse supernovae and augment current understanding of the explosion mechanism. A proof-of-principle study is also presented, using tailor-made simulations to demonstrate the viability of extracting the angular momentum distribution of nascent millisecond proto-neutron stars from their gravitational wave echoes. Thereafter, it considers the ramifications of failure to accurately capture proto-neutron star hydrodynamics in core-collapse supernova simulations; exploring the influence on the explosion mechanism of gravito-acoustic waves generated by convection in the proto-neutron star mantle. Finally, it ponders the impact of advances in multi-messenger astronomy and source modelling over the next twenty years on the understanding of core-collapse supernova physics.
", "doi": "10.7907/ZF1A-6394", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11587", "collection": "thesis", "collection_id": "11587", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312019-144001688", "primary_object_url": { "basename": "schmitz_denise_thesis_2019.pdf", "content": "final", "filesize": 899652, "license": "other", "mime_type": "application/pdf", "url": "/11587/1/schmitz_denise_thesis_2019.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A Perturbative Model for the Intrinsic Alignments of Galaxies", "author": [ { "family_name": "Schmitz", "given_name": "Denise Marie", "orcid": "0000-0001-6297-9312", "clpid": "Schmitz-Denise-Marie" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Hirata", "given_name": "Christopher M.", "orcid": "0000-0002-2951-4932", "clpid": "Hirata-C-M" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Dor\u00e9", "given_name": "Olivier P.", "orcid": "0000-0001-7432-2932", "clpid": "Dor\u00e9-O" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "TAPIR" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Intrinsic alignments (IA), correlations between the intrinsic shapes and orientations of galaxies on the sky, are both a significant systematic in weak lensing and a probe of the effect of large-scale structure on galactic structure and angular momentum. In the era of precision cosmology, it is thus especially important to model IA with high accuracy.
\r\n\r\nEfforts to use cosmological perturbation theory to model the dependence of IA on the large-scale structure have thus far been relatively successful. However, extant models have not been made fully self-consistent to arbitrary order in perturbation theory and do not consistently account for time evolution. In particular, advection of galaxies due to peculiar velocities alters the impact of IA, because galaxy positions when observed are generally different from their positions at the epoch when IA is believed to be set.
\r\n\r\nIn this work, we evolve the galaxy IA from the time of galaxy formation to the time at which they are observed, including the effects of this advection, and show how this process naturally leads to a dependence of IA on the velocity shear. We then incorporate this time evolution into a fully self-consistent perturbative formalism for a passively evolving IA model. We demonstrate this formalism first at second order as a proof of concept, then at third order for application to observationally relevant two-point correlations at one-loop order.
\r\n\r\nWe also discuss the implications of the time-evolved IA model for systematic errors in weak lensing as well as for studies of galaxy formation and evolution. We find that considering advection introduces nonlocality into the bispectrum, and that the degree of nonlocality represents the memory of a galaxy's path from the time of its formation to the time of observation. We discuss how this result can be used to constrain the redshift at which IA is determined and provide Fisher estimation for the relevant measurements using the example of SDSS-BOSS.
", "doi": "10.7907/HZ7G-1048", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11535", "collection": "thesis", "collection_id": "11535", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232019-145750803", "primary_object_url": { "basename": "Leethochawalit_Nicha_2019.pdf", "content": "final", "filesize": 3258747, "license": "other", "mime_type": "application/pdf", "url": "/11535/1/Leethochawalit_Nicha_2019.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Spectroscopy of Galaxies: Evolution of Escape Fractions, Metallicity Gradients and Stellar Metallicity", "author": [ { "family_name": "Leethochawalit", "given_name": "Nicha", "orcid": "0000-0003-4570-3159", "clpid": "Leethochawalit-Nicha" } ], "thesis_advisor": [ { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" } ], "thesis_committee": [ { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Ellis", "given_name": "Richard S.", "orcid": "0000-0001-7782-7071", "clpid": "Ellis-R-S" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "We use spectroscopic observations to investigate galaxies from the age of reionization to the peak of star formation, and the local universe. This thesis presents three projects to better understand characteristics of galactic feedback, i.e., how it regulates galactic gas flows.
\r\n\r\nWe used deep absorption line spectroscopy to estimate the escape fractions fesc of star-forming gravitationally-lensed galaxies at z \u2243 5. The approach is to measure the covering fraction of neutral hydrogen in a galaxy from the amount of non-ionizing UV radiation absorbed by low-ionization metal species. With the boost of signal by gravitational lensing, we observed four 4 < z < 5 star-forming galaxies with DEIMOS, doubling the sample size of existing observations at the redshift range. We found that the escape fractions across our sample varies from galaxy to galaxy and appear to have no significant evolution over time. We inferred a median absolute escape fraction of Lyman continuum photons of 19 \u00b1 6%. Accounting for possible biases and uncertainties, the absolute escape fraction could be reduced to no less than fesc,abs ~ 11%. Moreover, we spatially resolved and detected variation of escape fraction within a galaxy for the first time. The significant variations within the galaxy suggest that the escape fraction is governed by small-scale structure. We found a tentative anti-correlation between the star-formation rate (local or integrated) and the inferred escape fraction. This supports that the escape fraction is associated with the delay time after an episode of star formation. Feedback seems to be more effective in governing both a low SFR and a smaller HI covering fraction.
\r\n\r\nSpatially resolved spectroscopic observations of galaxies at the peak era of star-formation activities are effective in providing insight into the primitive disks. Specifically, we used metallicity gradients to provide constraints on the amount and extent of feedback produced in star-forming galaxies. We observed 15 star-forming galaxies at z ~ 2 with OSIRIS to obtain their kinematic properties and gas-phase metallicity gradients. With helps from AO correction and gravitational lensing, the typical spatial resolution in our study is less than a half-light radius of a typical L* galaxies at z \u2243 2. Combining with the sample in Jones et al., 2013, we approximately tripled the existing metallicity gradient measurements. We found a lower fraction of rotationally-supported systems than reported from larger kinematic surveys with coarser spatial resolution, which might be partially due to a our improved spatial resolution. We demonstrated that a high spatial resolution is crucial for an accurate diagnosis of the kinematic properties and dynamical maturity of z \u2243 2 galaxies.
\r\n\r\nAs for metallicity gradients, we found a much higher fraction of z \u2243 2 galaxies having weak or flat metallicity gradients than in previous studies. We correlated the metallicity gradient with the total metallicity and found that all galaxies with low total metallicities have flat gradients (< 0.1 dex per kpc-1). For galaxies with high metallicities ([N II]/H\u03b1 > 0.1), there is a divergence between isolated or rotationally-supported and dynamically-immature systems with the latter showing zero gradients irrespective of the integrated metallicity. The results indicate that relatively strong feedback (e.g. high mass loading factors or high SN energy output) is required in order to explain the majority of the observed flat gradients.
\r\n\r\nIn the second part of the thesis, we observed quiescent galaxies at z < 1 and archaeologically constrained galactic feedback via its cumulative influence on stellar metallicities. First, we present the stellar mass-[Fe/H] relationship in the Cl0024+17 galaxy cluster at z ~ 0.4. We derived the metalliticies via full spectrum stellar population synthesis modeling of individual quiescent galaxies. Our results provide the metallicities of the lowest galaxy mass (M* = 109.7M\u2299) at which individual stellar metallicity has been measured beyond the local universe. We found that the mass-[Fe/H] relationship evolves with the redshift at which the galaxy is observed today. Furthermore, we found an even stronger evolution of the mass-[Fe/H] relation with the time at which the galaxy formed (rather than the time at which it is observed). Galaxies that formed earlier have lower Fe abundance than galaxies that formed later.
\r\n\r\nLastly, we measured magnesium (Mg) abundances and extended the observed redshift to z ~ 0.55. We found that while the mass-[Fe/H] relation evolves significantly over the observed redshift range, the mass-[Mg/H] relation does not. This is due to the shorter star formation histories of quiescent galaxies at higher redshifts. Fe is mainly produced in Type Ia SN. It has a longer recycling time than Mg, which is mainly produced in core-collapse SN. Using core-collapse SN elements as a metal indicator lessens the complication of delayed recycling time and allows us to effectively use galactic chemical models with instantaneous recycling to quantify average outflows that these galaxies experience over their lifetime. We found that the average mass-loading factor \u03b7 is a power-law function of galaxy stellar mass, \u03b7 \u221d M*-0.21\u00b10.09, consistent with the results of other observational methods and with the predictions where outflow is caused by star formation feedback in turbulent disks.
", "doi": "10.7907/2GMR-3315", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11521", "collection": "thesis", "collection_id": "11521", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05152019-180537558", "type": "thesis", "title": "New Insights into Circumstellar Disk Evolution in the Era of ALMA", "author": [ { "family_name": "Barenfeld", "given_name": "Scott Alexander", "orcid": "0000-0001-5222-6851", "clpid": "Barenfeld-Scott-Alexander" } ], "thesis_advisor": [ { "family_name": "Carpenter", "given_name": "John M.", "orcid": "0000-0003-2251-0602", "clpid": "Carpenter-J-M" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" } ], "thesis_committee": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Carpenter", "given_name": "John M.", "orcid": "0000-0003-2251-0602", "clpid": "Carpenter-J-M" }, { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "In recent decades, it has become clear that solar systems such as our own form within the circumstellar disks of gas and dust that surround young stars. Thus, to understand how these solar systems come to be, it is necessary to study the conditions within these disks. Until recently, such studies have required a focus on intrinsically brighter and younger disks that are easier to observe. However, a full picture of planet formation requires the characterization of older disks to determine how these systems change over time. The unprecedented capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) now present the opportunity to study populations of these disks in detail for the first time. In this thesis, I present a study of over 100 such disks in the 5-11 Myr old Upper Scorpius OB Association (Upper Sco) using ALMA, as well as Keck Observatory, with the aim of comparing the properties of these systems to younger disks in order to shed new light on disk evolution.
\r\n\r\n\r\nFollowing background discussion on disks and their evolution, ALMA measurements of the continuum and CO line fluxes of these disks at 0.88 mm are reported in Chapter 3. The continuum fluxes are used to show that the majority of these systems contain less than 1 M\u2295 of dust. It is then shown that dust masses around these stars are on average a factor of \u223c4.5 lower than their younger counterparts in the Taurus star-forming region, placing important constraints on the mass evolution of these systems. Finally, constraints on depletion of gas in these disks are discussed using the CO measurements.
\r\n\r\n\r\nThe spatial distributions of the gas and dust within these Upper Sco disks are modeled in Chapter 4. The radial extents of gas and dust are measured and compared, with several systems showing evidence of the gas being more extended. The sizes of the dust disks are compared to younger systems, showing that these disks shrink by a factor of approximately three as they age. These results suggest that dust evolves from the outside-in within disks, perhaps through radial drift. Despite this evolution, dust disks in Upper Sco fall on the same correlation between size and millimeter luminosity as their younger counterparts. This implies a link between the radial structures of disks of different ages, perhaps indicating that these systems are composed of optically-thick dust substructure.
\r\n\r\n\r\nOf course, an understanding of planetary system formation would be incomplete without accounting for the presence of stellar companions, which are common around young stars and are expected to shorten disk lifetimes by truncating their sizes. As such, Chapter 5 presents a search for stellar companions in the Upper Sco disk sample analyzed in Chapters 3 and 4. Using adaptive optics imaging and aperture masking observations with the NIRC2 instrument on the Keck II telescope, stellar companions are identified in 27 of 112 systems. It is then shown that the companion fraction of systems with disks is lower than those without, confirming the harmful effects of stellar companions on disks seen in younger systems. However, the fraction of disk systems in Upper Sco with a close companion is shown to match that of younger disks in Taurus. This indicates that these effects occur within the first \u223cMyr of disk evolution, after which stellar companions have little to no effect. Additionally, while the millimeter luminosities of disks with stellar companions are observed to be lower than those around single stars in Taurus, there exists no such difference in Upper Sco. This provides further support for outside-in dust evolution, as the shrinking of disks around single stars would cause them by the 5-11 Myr age of Upper Sco to match the sizes and brightnesses of truncated disks in binary systems.
\r\n\r\n\r\nTaken together, the results presented in this thesis show the masses and radial extents of the dust-component of circumstellar disks decrease with age. This thesis concludes by summarizing these results and discussing their link within a scenario of outside-in dust evolution involving radial drift and dust substructure. To close, potential avenues are presented to continue the study of disk evolution with ALMA.
", "doi": "10.7907/5DK0-H549", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11518", "collection": "thesis", "collection_id": "11518", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05142019-161824896", "primary_object_url": { "basename": "Final_Caltech_Thesis_Kung-Yi_Su.pdf", "content": "final", "filesize": 13734372, "license": "other", "mime_type": "application/pdf", "url": "/11518/1/Final_Caltech_Thesis_Kung-Yi_Su.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Stellar Feedback, AGN Feedback and Fluid Microphysics in Galaxy Evolution", "author": [ { "family_name": "Su", "given_name": "Kung-Yi", "orcid": "0000-0003-1598-0083", "clpid": "Su-Kung-Yi" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Martin", "given_name": "D. Christopher", "clpid": "Martin-D-Christopher" }, { "family_name": "Steidel", "given_name": "Charles C.", "clpid": "Steidel-C-C" }, { "family_name": "Kirby", "given_name": "Evan N.", "clpid": "Kirby-E-N" }, { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "TAPIR" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Understanding how the baryonic physics affects the formation and evolution of galaxies is one of the most critical questions in modern astronomy. Significant progress in understanding stellar feedback and modeling them explicitly in simulations have made it possible to reproduce a wide range of observed galaxy properties. However, there are still various pieces of missing physics and uncertainties in galaxies of different mass range.
\r\n\r\nIn this thesis, I will explore these missing pieces in baryonic physics on top of the Feedback in Realistic Environments (FIRE) stellar feedback in the cosmological hydrodynamic zoom-in simulations (FIRE-2 suite) and isolated galaxy simulations. These high-resolution simulations with FIRE physics capture multi-phase realistic interstellar medium (ISM) with gas cooling down to 10K, and star formations in dense clumps in giant molecular clouds. They are, therefore, an ideal tool for investigating the missing pieces in baryonic physics.
\r\n\r\nIn the first part of the thesis, Chapter 2, I will focus on the discrete effects of stellar feedback like individual supernovae, hypernovae, and initial mass function (IMF) sampling in dwarfs (109-1010 M\u2299). These discrete processes of stellar feedback can have maximum effects on the small galaxies without being averaged out. I will show that the discretization of supernovae (SNe) is absolutely necessary, while the effects from IMF sampling and hypernovae (HNe) is not apparent, due to the strong clustering nature of star formation.
\r\n\r\nIn the second part of the thesis, Chapter 3-4, I will focus on fluid microphysics, exploring their effects on galaxy properties and their interplay with stellar feedback in sub-L* galaxies. I will demonstrate that, once the stellar feedback is explicitly implemented as FIRE stellar feedback model, fluid microphysics such as magnetic fields, conduction, and viscosity only have minor effects on the galaxy properties like star formation rate (SFR), phase structure, or outflows. Stellar feedback also strongly alters the amplifications and morphology of the magnetic fields, resulting in much more randomly-oriented field lines. However, despite the stellar feedback, the amplification of magnetic fields in ISM gas is primarily dominated by flux-freezing compression.
\r\n\r\nIn the final part of my thesis, I focus on the massive cluster ellipticals of 1012-1014 M\u2299, where the physical mechanisms that regulate the observation-inferred cooling flows are highly uncertain -- the classic \"cooling flow problem\". I showed that solutions in the literature not associated with an active galactic nucleus (AGN), including stellar feedback, the cosmic ray from stellar feedback, magnetic fields, conduction, and morphological quenching, cannot possibly quench the galaxies, mostly because of the insufficient energy and the limited size of the affected region. After ruling out the non-AGN feedback solutions to the cooling flow problem, I will go into the most accessible, and perhaps promising solution: \"AGN feedback\", exploring the generic classes of AGN feedback models proposed in the literature. I am going to show that enhancing turbulence and injecting cosmic ray are probably the most important aspects of AGN feedback in galaxy quenching. Since they provide non-thermal pressure support that stably suppresses the core density, they can stably reduce the cooling flows without overheating the galactic cores.
", "doi": "10.7907/RQG9-NT50", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11517", "collection": "thesis", "collection_id": "11517", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05142019-152545186", "primary_object_url": { "basename": "OrrThesis_Final.pdf", "content": "final", "filesize": 9182630, "license": "other", "mime_type": "application/pdf", "url": "/11517/1/OrrThesis_Final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Spatially Resolved Star Formation in Cosmological Zoom-in Simulations: Understanding the Role of Feedback in Scaling Relations", "author": [ { "family_name": "Orr", "given_name": "Matthew Edward", "orcid": "0000-0003-1053-3081", "clpid": "Orr-Matthew-Edward" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "clpid": "Scoville-N-Z" }, { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" }, { "family_name": "Steidel", "given_name": "Charles C.", "clpid": "Steidel-C-C" }, { "family_name": "Armus", "given_name": "Lee", "clpid": "Armus-L" } ], "local_group": [ { "literal": "TAPIR" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "To understand the night sky is to understand how galaxies form their stars. Cosmological zoom-in simulations, which self-consistently evolve a small number of galaxies at very high resolution by embedding them within a fully cosmological box, have evolved over the last 25 years to a level of realism where they can begin to tackle questions of spatially resolved star formation within galaxies. Whereas a decade ago simulations faced difficulty in matching even global properties of observed galaxies (e.g., the ratio of stellar mass to total halo mass), the state of the art is now able to meaningfully recover resolved quantities in galaxies that were not put into the simulations by hand (e.g., the Kennicutt-Schmidt star formation scaling relation).
\r\n\r\nThe research presented in this thesis seeks to understand how the physics of star formation and stellar feedback from massive stars shape and regulate the interstellar medium (ISM) within galaxies. Particularly, the focus lies on the scale of the largest coherent structures in galaxies -- the disk scale height. To explore these physics, the cosmological zoom-in simulations of the Feedback in Realistic Environments (FIRE) project (Hopkins et al. 2014, 2018) are used.
\r\n\r\nThe chapters of this thesis explore various topics in spatially resolved star formation, including: the Kennicutt-Schmidt relation (Schmidt 1959, Kennicutt 1998), an empirical relation between gas surface density and star formation rates, in the FIRE-1 simulations (Orr et al. 2018), including an examination what set the extent of the star-forming disks in the simulations (i.e., what causes star formation to fire up in the outskirts); an examination of the observational method of analyzing stacks of galaxy observations, finding that temporal variations in spatially resolved star formation rates within individual galaxies were more than enough to bias stacking analysis of star formation rate profiles; a semi-analytic model of non-equilibrium star formation rates, relating to the competition between the feedback timescale associated with star formation and local dynamical times (Orr et al. 2019), which explores this as a source of scatter in the Kennicutt-Schmidt relation; and finally, investigating how gas velocity dispersions and star formation rates relate in FIRE-2 Milky Way-mass disk galaxies, exploring whether or not feedback is primarily driving the velocity dispersions in galaxies, and how quickly local patches can self-regulate with star formation (Orr et al. in prep.).
", "doi": "10.7907/WE1D-5586", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11708", "collection": "thesis", "collection_id": "11708", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-111646558", "primary_object_url": { "basename": "MYG_Thesis.pdf", "content": "final", "filesize": 10002492, "license": "other", "mime_type": "application/pdf", "url": "/11708/1/MYG_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "The Role of Stellar Feedback in Star Cluster Formation", "author": [ { "family_name": "Grudi\u0107", "given_name": "Michael Yvan", "orcid": "0000-0002-1655-5604", "clpid": "Grudi\u0107-Michael-Yvan" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Murray", "given_name": "Norman", "clpid": "Murray-N-M" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "A methodology for numerical magnetohydrodynamics simulations of star cluster formation, accounting for all mechanisms of stellar feedback from massive stars, is developed and used to address a range of problems regarding the formation of stars and star clusters in giant molecular clouds (GMCs). These studies culminate in a new theoretical framework that connects properties of GMCs to those of the star clusters that form in them.
\r\n\r\nThe simulation methodology is established and tested, and the problem of the star formation efficiency (SFE) of molecular clouds is addressed. It is found that SFE is set by the balance of feedback and gravity, with very weak dependence upon other factors. A simple dimensional scaling law with cloud surface density emerges from the complex interplay of different feedback physics. Parameter space is found where feedback must fail, and the SFE is high, and the implications of this prediction are explored.
\r\n\r\nThe star clusters formed in the simulations are found to resemble observed young, massive star clusters in the form of their surface brightness profiles, leading to the hypothesis that this structure is a result of the star formation process. It is shown that the shallow, power-law density profiles characteristic of young clusters is predicted by the hierarchical star formation scenario. It is shown that the SFE law, when coupled to an analytic cloud collapse model, predicts that gas should be exhausted by highly-efficient star formation at a stellar surface density of \u223c 105 \u2212 106 Msun pc-2, consistent with the maximum observed.
\r\n\r\nA new suite of simulations is developed to specifically model GMCs in the Milky Way. It is found that the picture of feedback-disrupted star formation is able to account for both the normalization and the scatter in the measured SFE of GMCs in the Milky Way, the first theory to do so.
\r\n\r\nThe uncertainty in the simulated SFE due to the choice of feedback prescription is quantified, by running a controlled methods study of several different prescriptions in the literature. In the cloud model simulated, the choice of prescription affects the simulated SFE at the factor of \u223c 3 level, explaining discrepancies in the literature and identifying the small-scale details of massive star formation as the main uncertainty in cluster formation simulations.
\r\n\r\nFinally, the simulation suite is extended to model massive GMCs in local spiral galaxies, and to simulate 10 random realizations at each point in parameter space, mapping out the stochastic nature of star cluster formation in GMCs. A model is calibrated to the simulation results, taking the cloud bulk properties as input parameters, and predicting the detailed properties of the star clusters formed in it. A star cluster catalogue is synthesized from observed GMCs in M83, and good agreement is found with observed star cluster properties, including the fraction of stars in bound clusters, the maximum cluster mass, and the distribution of cluster sizes.
", "doi": "10.7907/7FBG-TE87", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11264", "collection": "thesis", "collection_id": "11264", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11042018-161928279", "type": "thesis", "title": "Fundamental Physics in the Era of Gravitational-Wave Astronomy: The Direct Measurement of Gravitational-Wave Polarizations and Other Topics", "author": [ { "family_name": "Isi Ba\u00f1ales", "given_name": "Maximiliano", "orcid": "0000-0001-8830-8672", "clpid": "Isi-Ba\u00f1ales-Maximiliano" } ], "thesis_advisor": [ { "family_name": "Weinstein", "given_name": "Alan Jay", "orcid": "0000-0002-0928-6784", "clpid": "Weinstein-Alan-J-Physics" } ], "thesis_committee": [ { "family_name": "Chen", "given_name": "Yanbei", "orcid": "0000-0002-9730-9463", "clpid": "Chen-Yanbei" }, { "family_name": "Weinstein", "given_name": "Alan Jay", "orcid": "0000-0002-0928-6784", "clpid": "Weinstein-Alan-J-Physics" }, { "family_name": "Patterson", "given_name": "Ryan B.", "orcid": "0000-0002-5787-9517", "clpid": "Patterson-R-B" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "LIGO" }, { "literal": "div_pma" } ], "abstract": "In this thesis, I explore several avenues for learning about fundamental physics from gravitational-wave (GW) observations. In particular, I focus on the phenomenological study of basic properties of GWs in ways that require minimal assumptions about the underlying nature of gravity. I place a special emphasis on GW polarizations, but also consider their speed and possible dispersion. To constrain possible modifications to general relativity, I develop data-analysis frameworks to measure these properties with both transient and persistent signals detected by ground-based detectors. This includes, among other results, the analysis of two LIGO and Virgo compact-binary detections, GW170814 and GW170817, to produce the first direct observational statements about the local geometry of GW polarizations. I also present constraints on the potential amplitude of nontensorial monochromatic signals from 200 known pulsars in the Milky Way and describe in detail the methods used to obtain them. Because stochastic signals will be a great resource for studying GW properties, I also carefully review the assumptions that go into standard stochastic analyses and explore their applicability beyond general relativity, concluding that those measurements will have to be interpreted carefully to make meaningful statements about corrections to Einstein\u2019s theory. Besides the properties of the waves themselves, I also study the prospect for using GWs as a means to uncover signatures of new ultralight bosons\u2014an exciting possibility that could bring particle physics into the reach of GW astronomy. I explore the potential of current and future detectors to detect these conjectured particles, concluding that third-generation instruments are certain to place theoretically interesting constraints. Because little can be done in the absence of signals, I also propose data-analysis methods to improve LIGO and Virgo\u2019s chances of detecting both transient and continuous signals. The latter have now been used to diagnose real detection candidates on several occasions.
", "doi": "10.7907/S15R-3495", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11170", "collection": "thesis", "collection_id": "11170", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09032018-174351545", "type": "thesis", "title": "Modeling Rapidly Fading Supernovae as Nickel-Free Core-Collapse Explosions of Extended Helium Stars", "author": [ { "family_name": "Kleiser", "given_name": "Io K.", "orcid": "0000-0001-5332-1156", "clpid": "Kleiser-Io-K" } ], "thesis_advisor": [ { "family_name": "Phinney", "given_name": "E. Sterl", "clpid": "Phinney-E-S" }, { "family_name": "Kasen", "given_name": "Daniel", "clpid": "Kasen-D" } ], "thesis_committee": [ { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" }, { "family_name": "Steidel", "given_name": "Charles C.", "clpid": "Steidel-C-C" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "clpid": "Kulkarni-S-R" }, { "family_name": "Kirby", "given_name": "Evan N.", "clpid": "Kirby-E-N" }, { "family_name": "Phinney", "given_name": "E. Sterl", "clpid": "Phinney-E-S" }, { "family_name": "Kasen", "given_name": "Daniel", "clpid": "Kasen-D" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Supernovae are the engines of the universe, pulling material out of the furnaces of stars and spewing it out into their galaxies. As some of the most powerful explosions since the Big Bang, they influence not only the chemical but also mechanical evolution of the galaxies they inhabit. They induce star formation and produce the building blocks of planets, organisms, and ultimately, civilizations. Understanding the connections between the supernovae we observe and the stars that would have produced them is a critical piece of understanding this process.
\r\n\r\nUnfortunately, we rarely have the ability to observe the progenitor stars of supernovae directly; it is usually difficult to predict when a given star will explode, and most are in galaxies too distant to allow observation of individual stars. Instead, we typically must leverage our understanding of the explosions themselves to reveal the nature of the stars that produced them. Using analytical and numerical calculations, it is possible to predict the supernovae from certain types of stars and work backwards.
\r\n\r\nIn this thesis, we present a new model for previously elusive rapidly fading supernovae, which we believe are due to the core-collapse explosions of massive stars inside extended hydrogen-free envelopes or previously ejected mass shells. This model requires not only pre-explosion stellar radii of unprecedented size for hydrogen-free stars but also a lack of radioactive nickel, which is usually present in supernovae. We show our process from simple toy models to self-consistent explosions of stellar models and compare our results to existing rapidly fading supernovae. Understanding these unusual transients will shed light on the many possible ways stars behave shortly before death and also may be critical for understanding the population of core-collapse supernovae as a whole.
", "doi": "10.7907/5YEJ-EX71", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11168", "collection": "thesis", "collection_id": "11168", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09022018-124448728", "primary_object_url": { "basename": "Rankin_Jamie_2019.pdf", "content": "final", "filesize": 37042966, "license": "other", "mime_type": "application/pdf", "url": "/11168/1/Rankin_Jamie_2019.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Voyager 1 Observations of Galactic Cosmic Ray Anisotropies in the Local Interstellar Medium", "author": [ { "family_name": "Rankin", "given_name": "Jamie Sue", "orcid": "0000-0002-8111-1444", "clpid": "Rankin-Jamie-Sue" } ], "thesis_advisor": [ { "family_name": "Stone", "given_name": "Edward C.", "orcid": "0000-0002-2010-5462", "clpid": "Stone-E-C" } ], "thesis_committee": [ { "family_name": "Stone", "given_name": "Edward C.", "orcid": "0000-0002-2010-5462", "clpid": "Stone-E-C" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Patterson", "given_name": "Ryan B.", "orcid": "0000-0002-5787-9517", "clpid": "Patterson-R-B" } ], "local_group": [ { "literal": "Space Radiation Laboratory" }, { "literal": "div_pma" } ], "abstract": "Since crossing the heliopause on August 25, 2012, Voyager 1 has observed reductions in galactic cosmic ray counting rates caused by a time-varying depletion of particles with pitch angles near 90-deg, while intensities of particles with other pitch angles remain unchanged. Between late 2012 and mid-2017, three large-scale, durable events occurred, lasting roughly 100 to 630 days. Omnidirectional and directional data from the Cosmic Ray Subsystem's high energy telescopes are used to report observations of the cosmic ray intensity variations. Omnidirectional (>20 MeV) proton-dominated measurements show up to a ~3.8% intensity reduction. Bi-directional (>70 MeV) and unidirectional (~18 to ~70 MeV) proton-dominated measurements are analyzed using data taken from various spacecraft orientations, including during magnetometer roll calibrations and 70-deg-offset maneuvers. The anisotropy is characterized as a \"notch\" in an otherwise uniform pitch-angle distribution of varying depth and width centered about 90-deg in pitch angle space. The notch averages ~22-deg wide and ~15% deep -- signifying a depletion region that is broad and shallow. However, electron observations reveal that there is only a weak, at most, evidence of pitch angle anisotropy in cosmic-ray electrons with energies of ~3 to ~105 MeV, indicating that the generation of the notch or its evolution differs between electrons and protons, or varies with rigidity. There are indications that the anisotropy is formed by a combination of magnetic trapping and adiabatic cooling in associated shocks or compression regions.\r\n", "doi": "10.7907/WX3S-1D86", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:10423", "collection": "thesis", "collection_id": "10423", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09082017-170618214", "type": "thesis", "title": "Unveiling the Physical Conditions in Star-Forming Galaxies at the Peak of Galaxy Assembly", "author": [ { "family_name": "Strom", "given_name": "Allison Leigh", "orcid": "0000-0001-6369-1636", "clpid": "Strom-Allison-Leigh" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "thesis_committee": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Kirby", "given_name": "Evan N.", "orcid": "0000-0001-6196-5162", "clpid": "Kirby-E-N" }, { "family_name": "Soifer", "given_name": "B. Thomas", "clpid": "Soifer-B-T" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Galaxies at the peak of cosmic star formation (z~2-3) exhibit significantly higher star formation rates and gas fractions at fixed stellar mass than nearby galaxies. These z~2-3 galaxies are also distinct in terms of their nebular spectra, reflecting important differences not only in the physical conditions of their interstellar medium (e.g., electron density and gas-phase metallicity), but also in the details of their massive stellar populations. Jointly observing galaxies' HII regions, at rest-UV and rest-optical wavelengths, and massive stars, at rest-UV wavelengths, is central to constructing a framework for understanding the differences between z~2-3 and z~0 star-forming galaxies and also vital for self-consistently explaining the trends observed in the high-z population.
\r\n\r\nThis thesis presents the main results from the near-infrared (NIR) component of the Keck Baryonic Structure Survey (KBSS), a targeted spectroscopic survey of z~2-3 galaxies that uniquely combines observations in the rest-UV (1000-2000\u00c5) and rest-optical (3500-7500\u00c5) bandpasses. The NIR spectroscopic campaign conducted using Keck/MOSFIRE, described in Chapter 2, includes observations over 1200 high-z galaxies and represents one of the largest samples of high-quality rest-optical spectra of z~2-3 galaxies ever assembled. These measurements offer new insights regarding the physical conditions in galaxies forming during one of the most active periods in the universe's history.
\r\n\r\nChapter 3 describes the rest-optical spectra of ~380 KBSS galaxies at z~2-2.7 and shows that the primary difference between HII regions in z~2.3 galaxies and those at z~0 is an enhancement in the degree of nebular excitation. KBSS galaxies are also 10 times more massive than z~0 galaxies with similar ionizing spectra and have higher gas-phase N and O abundances at fixed excitation. These results indicate the presence of harder ionizing radiation fields at fixed gas-phase enrichment relative to typical z~0 galaxies, consistent with Fe-poor stellar population models that include massive binaries.
\r\n\r\nChapter 4 builds on this analysis to develop a new technique for determining the physical conditions in individual high-z galaxies -- independent of diagnostics tuned to local calibration samples. This method produces self-consistent measurements of the chemical enrichment and excitation conditions in ~150 galaxies at z~2-2.7.
\r\n\r\nTogether, these results provide compelling evidence that the distinct chemical abundance patterns observed in z~2-3 star-forming galaxies result from systematic differences in their star formation histories relative to galaxies with similar stellar masses today. The thesis concludes by considering the importance of accounting for differences in galaxies' past star formation when interpreting spectroscopic observations and briefly discusses opportunities for extending the framework for analyzing high-z galaxies presented here to future studies of star-forming galaxies throughout cosmic time.
", "doi": "10.7907/Z9N58JJN", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10881", "collection": "thesis", "collection_id": "10881", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05142018-155029272", "primary_object_url": { "basename": "Guszejnov_thesis.pdf", "content": "final", "filesize": 5939250, "license": "other", "mime_type": "application/pdf", "url": "/10881/1/Guszejnov_thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "On the Origin of Scales and Scaling Laws in Star Formation", "author": [ { "family_name": "Guszejnov", "given_name": "D\u00e1vid", "orcid": "0000-0001-5541-3150", "clpid": "Guszejnov-D\u00e1vid" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "orcid": "0000-0002-0438-3323", "clpid": "Scoville-N-Z" }, { "family_name": "Keres", "given_name": "Dusan", "clpid": "Keres-Dusan" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" } ], "local_group": [ { "literal": "TAPIR" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Stars are the fundamental objects of astrophysics and their formation is a key process that influences the evolution of galaxies and planets, and the development of life. Understanding how stars form is crucial even for seemingly unconnected fields of astronomy, as the interpretation of observed starlight from galaxies (even as a background) and other unresolved sources relies on our understanding of star formation. There is no comprehensive theory of star formation, despite intense effort on both the theoretical and observational sides, due to the complicated, non-linear physics involved (e.g., magnetohydrodynamics, gravity, radiation) and the enormous dynamic range of the problem. My goal has been to identify the role different physical processes (e.g., turbulence, feedback) play in star formation.
\r\n\r\nUsing the semi-analytical framework that I developed, I found that a large number of star formation models are inherently sensitive to the initial conditions of the progenitor clouds (e.g., temperature). This led to another study where I predicted the expected variation of the initial mass function (IMF) of stars in a Milky-Way-like galaxy for different star formation models. I showed that IMF models where the peak is either set by turbulent properties or cooling physics (using an effective equation of state) are unable to reproduce the universal IMF of the Milky Way. I also utilized my semi-analytical tools to predict higher-order statistics of star formation: stellar correlation, multiplicity and the companion mass distribution for binaries. I showed that due to observational biases all explored models could roughly reproduce the observed multiplicity and companion mass distributions. This means that observations are currently unable to differentiate between most models.
\r\n\r\nWhile working on these projects I found that several scaling relations (e.g., the slopes of IMF, stellar correlation function and gas column density distribution) are insensitive to our choice of physical model; they are universal. Inspired by this I developed an analytic model that I used to show that scale-free structure formation inherently leads to these scaling relations. This provides a deep physical reason why the mass functions and correlation functions of very different systems (e.g., stars, protostellar cores, star clusters, Dark matter halos) follow roughly the same power-law relations.
\r\n\r\nMy previous findings with my semi-analytical models indicated that isothermal collapse would lead to an infinite fragmentation cascade, i.e. there is no inherent low-mass cut-off. Part of the literature supports these findings and claims that additional physics is needed to imprint a mass scale into the problem, but there are theoretical models and simulations that claim that such a cut-off exists. Using GIZMO, a fully adaptive, meshless MHD code, I have carried out a convergence study and have shown that the isothermal fragmentation cascade continues to ever smaller scales without limit.
", "doi": "10.7907/WZ2G-R643", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10929", "collection": "thesis", "collection_id": "10929", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232018-173148982", "primary_object_url": { "basename": "ma_xiangcheng_2018.pdf", "content": "final", "filesize": 14718465, "license": "other", "mime_type": "application/pdf", "url": "/10929/1/ma_xiangcheng_2018.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Understanding Galaxy Formation and Evolution with Realistic Simulations", "author": [ { "family_name": "Ma", "given_name": "Xiangcheng", "orcid": "0000-0001-8091-2349", "clpid": "Ma-Xiangcheng" } ], "thesis_advisor": [ { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" } ], "thesis_committee": [ { "family_name": "Steidel", "given_name": "Charles C.", "clpid": "Steidel-C-C" }, { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" }, { "family_name": "Kirby", "given_name": "Evan N.", "clpid": "Kirby-E-N" }, { "family_name": "Quataert", "given_name": "Eliot", "clpid": "Quataert-Eliot" } ], "local_group": [ { "literal": "TAPIR" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Understanding the formation and evolution of galaxies from the Big Bang to the present day is one of the most important questions in modern astronomy. The tremendous amount of observational data accumulated in the past decade that probe various properties of galaxies across cosmic time demand a more detailed theoretical understanding of galaxy formation and evolution.
\r\n\r\nIn this thesis, I will investigate several open question in this field using state-of-the-art cosmological hydrodynamic zoom-in simulations of galaxy formation from the Feedback in Realistic Environments (FIRE) suite. These high-resolution simulations (10-104M\u2299, 0.1-10pc) include realistic models of the multi-phase ISM, star formation, and stellar feedback and explicitly capture gas cooling down to 10 K, star formation in dense clumps in giant molecular clouds, and feedback coupling on the smallest resolved scales. These simulations are powerful tools for studying the key physics governing galaxy formation and evolution and understanding the detailed observations of galaxy properties.
\r\n\r\nThe first half of this thesis presents three studies on galactic chemical evolution. Chapter 2 focuses on the origin and evolution of the galaxy mass-metallicity relation (MZR), one of the fundamental properties of galaxies. I will show that the FIRE simulations broadly agree with the observed galaxy MZR from z = 0-3. The slope of the MZR is mainly driven by the metal retention fraction in low-mass galaxies, while the amount of redshift evolution of the MZR is mostly determined by the star formation histories of galaxies. Chapter 3 attempts to understanding the diversity of gas-phase metallicity gradients found in intermediate-redshift (z ~ 0.6-3) galaxies. I will show that the metallicity gradient in a galaxy varies on small timescales driven by bursty star formation and feedback cycle at early times, naturally resulting in the observed diversity of metallicity gradients in z ~ 2 galaxies. The metallicity gradient only reflects the instantaneous dynamics of a galaxy. Chapter 4 will study the structure, stellar age and metallicity gradients, and formation history of Milky Way (MW)-like disk galaxies. At high redshift, star formation happens in a chaotic, bursty mode, which eventually forms a nearly spherical structure by z = 0. Since z \u227e 1, a stable gas disk emerged and stars formed in that disk thereafter. The thickness of the gas disk decreases with time due to lowering gas fraction. Stars formed earlier in this disk are kinematically heated to a thicker, flaring disk. Such a formation history leads to the age and stellar metallicity gradients consistent with what observed in the MW disk.
\r\n\r\nThe second half of this thesis focuses on galaxy formation in the first billion years of the Universe, known as the reionization era. Chapters 5 and 6 study the escape fraction of ionizing photons from galaxies at z \u2265 5, which is an important, yet poorly constrained parameter for understanding the reionization history. Most ionizing photons are emitted by the youngest stellar populations in the galaxy, which are usually embedded in their 'birth clouds'. Stellar feedback is required to clear these clouds in a few Myr before ionizing photons are allowed escape. In the meanwhile, the ionizing photon budget decreases rapidly as the most massive stars start to die. The competition of timescales between feedback and stellar evolution is thus the most important physics determines fesc. I will show that canonical single-star stellar population models such as STARBURST99 generally yield a fesc far below what is required for cosmic reionization. Binary models, in contrast, produce more ionizing photons at late times than single-star models and thus lead to a much higher fesc. Chapter 7 presents a new suite of high-resolution cosmological zoom-in simulations of z \u2265 5 galaxies that contains thousands of halos at any time in all zoom-in regions. I will present the stellar mass-halo mass relation, SFR-Mhalo relation, stellar mass-magnitude relation, stellar mass functions, and multi-band luminosity functions at z = 5-12. These prediction agree well with current observational constraints and can be further tested by future observations with the James Webb Space Telescope. Using these new simulations, Chapter 8 studies the morphology and size evolution of galaxies at z \u2265 5. I will show that the rest-frame UV light from z \u2265 5 galaxies is usually dominated by one or several star-forming clumps that are intrinsically bright and small. Current observations with moderate surface brightness limits tend to only pick up the intrinsically small galaxies or individual clumps but miss the diffuse light in the galaxies. Such a selection effect is likely to result in the extremely small sizes claimed for the faint galaxies in the Hubble Frontier Fields.
", "doi": "10.7907/MX2D-6P63", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10347", "collection": "thesis", "collection_id": "10347", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07212017-001528247", "primary_object_url": { "basename": "Sherwood_Richers_2017.pdf", "content": "final", "filesize": 21616570, "license": "other", "mime_type": "application/pdf", "url": "/10347/1/Sherwood_Richers_2017.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Neutrino Radiation Transport and Other Topics in High Energy Density Astrophysics", "author": [ { "family_name": "Richers", "given_name": "Sherwood Andrew, III", "orcid": "0000-0001-5031-6829", "clpid": "Richers-Sherwood-Andrew-III" } ], "thesis_advisor": [ { "family_name": "Ott", "given_name": "Christian D.", "clpid": "Ott-C-D" } ], "thesis_committee": [ { "family_name": "Ott", "given_name": "Christian D.", "clpid": "Ott-C-D" }, { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" }, { "family_name": "Filippone", "given_name": "Bradley W.", "clpid": "Filippone-B-W" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "clpid": "Kulkarni-S-R" }, { "family_name": "Kasen", "given_name": "Daniel", "clpid": "Kasen-D" } ], "local_group": [ { "literal": "TAPIR" }, { "literal": "div_pma" } ], "abstract": "Neutron star mergers and the collapse of massive stars result in some of the universe\u2019s most violet explosions. However, the detailed mechanisms behind all of these astrophysical explosions remain elusive. Their strongly nonlinear and complicated nature makes them difficult and expensive to simulate, and the properties of matter in these extreme conditions are poorly constrained. I use a variety of computational tools to understand the detailed mechanisms behind both types of events.
\r\n\r\nI describe my relativistic time-independent multidimensional Monte Carlo neutrino radiation transport code Sedonu that provides an accurate account of the neutrino radiation fields and the interaction with neutrinos and background fluid. Though Sedonu calculations are time-independent, I demonstrate their utility in dynamical general relativistic variable Eddington tensor radiation hydrodynamics simulations.
\r\n\r\nI apply Sedonu to simulations of accretion disks following neutron star mergers to demonstrate that more realistic disk cooling and neutrino-driven mass ejection rates are larger than is predicted using approximate transport methods. I also reinforce that neutrino pair annihilation from these disk configurations is unlikely to be able to energize a gamma-ray burst jet.
\r\n\r\nI subject Sedonu to the first thorough comparison of Boltzmann neutrino radiation transport methods in multiple spatial dimensions in the context of core-collapse supernovae. The comparisons with the other highly accurate discrete ordinates-based transport scheme show remarkably similar results, verifying the accuracy of both methods and underscoring the importance of numerical fidelity.
\r\n\r\nI perform the first broad parameter study on how different descriptions of dense nuclear matter and star rotation rates influence the dynamics of, and hence gravitational waves from, the bounce and early post-bounce phase of rapidly rotating core collapse supernovae. Using the results of 1824 two-dimensional general relativistic core-collapse simulations, I demonstrate that the equation of state is unlikely to be constrained by LIGO observations. I show that the effect of the equation of state on the gravitational wave frequency can be described by a single universal relation.
\r\n\r\nFinally, I use results of three-dimensional general relativistic magnetohydrodynamics simulations of rapidly rotating core collapse to demonstrate that the polar magnetic structures that form are destroyed by a magnetohydrodynamic kink instability.
", "doi": "10.7907/Z9PC30JH", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10235", "collection": "thesis", "collection_id": "10235", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312017-154839851", "primary_object_url": { "basename": "Antonija_Oklopcic_thesis_2017_final.pdf", "content": "final", "filesize": 8580590, "license": "other", "mime_type": "application/pdf", "url": "/10235/3/Antonija_Oklopcic_thesis_2017_final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Radiative Processes in Astrophysical Gases: From the Intergalactic and Interstellar Medium to Exoplanetary Atmospheres", "author": [ { "family_name": "Oklop\u010di\u0107", "given_name": "Antonija", "orcid": "0000-0002-9584-6476", "clpid": "Oklop\u010di\u0107-Antonija" } ], "thesis_advisor": [ { "family_name": "Hirata", "given_name": "Christopher M.", "orcid": "0000-0002-2951-4932", "clpid": "Hirata-C-M" } ], "thesis_committee": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Readhead", "given_name": "Anthony C. S.", "orcid": "0000-0001-9152-961X", "clpid": "Readhead-A-C-S" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Hirata", "given_name": "Christopher M.", "orcid": "0000-0002-2951-4932", "clpid": "Hirata-C-M" } ], "local_group": [ { "literal": "TAPIR" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "This thesis presents theoretical investigations in three areas of astrophysics, all related to radiative processes and interactions between stellar radiation and gaseous media in the Universe, ranging from the intergalactic and interstellar medium to planetary atmospheres.
\r\n\r\nPart I of the thesis consists of two independent investigations in which we study the effects of stellar feedback in high-redshift environments. The topic of Chapter 2 is the intergalactic medium (IGM) in the epoch just after the formation of the first stars in the Universe, but before the cosmic reionization was completed. This epoch is of great interest for the ongoing and future experiments aimed at observing the neutral IGM via the redshifted 21 cm line of hydrogen. We study the effects of resonant scattering of Lyman-\u03b1 photons produced by early stars on the structure of temperature fluctuations in the IGM. In Chapter 3, we use cosmological hydrodynamic simulations of galaxy evolution to study the effects of stellar feedback on the clumpy structure of star-forming galaxies at z ~ 2. Observations of high-redshift galaxies show that their morphology is often dominated by a few giant clumps of intense star formation, but the nature and the importance of these clumps for the evolution of their host galaxies are uncertain. We present a detailed analysis of the properties of giant clumps in a high-redshift simulated galaxy from the FIRE project.
\r\n\r\nPart II of the thesis is devoted to the effects of Raman scattering of stellar radiation in the atmospheres of extrasolar planets. Spectral signatures of Raman scattering imprinted in the geometric albedo spectrum of a gaseous planet carry information about the properties of the planet's atmosphere---its composition, temperature, and the radiation-penetration depth. In Chapter 5, we present the results of radiative transfer calculations including the treatment of Raman scattering for different types of planetary atmospheres and analyze the feasibility of detecting the spectral signatures of Raman scattering in nearby exoplanets. The structure and the intensity of Raman spectral features depends on both the atmospheric properties and the shape of the stellar spectrum irradiating the atmosphere. In Chapter 6, we analyze the diversity of Raman features in the geometric albedo spectra of planets hosted by different types of stars.
", "doi": "10.7907/Z9GM85CS", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10294", "collection": "thesis", "collection_id": "10294", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06052017-232351957", "type": "thesis", "title": "Unveiling the Structure of Active Galactic Nuclei with Hard X-ray Spectroscopy", "author": [ { "family_name": "Balokovic", "given_name": "Mislav", "orcid": "0000-0003-0476-6647", "clpid": "Balokovic-Mislav" } ], "thesis_advisor": [ { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" } ], "thesis_committee": [ { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "orcid": "0000-0002-0438-3323", "clpid": "Scoville-N-Z" }, { "family_name": "Readhead", "given_name": "Anthony C. S.", "orcid": "0000-0001-9152-961X", "clpid": "Readhead-A-C-S" }, { "family_name": "Harrison", "given_name": "Fiona A.", "orcid": "0000-0003-2992-8024", "clpid": "Harrison-F-A" } ], "local_group": [ { "literal": "Space Radiation Laboratory" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Despite the long history of studies of active galactic nuclei (AGN), details of the structure of the accretion flow onto supermassive black holes are far from clear. Work presented in this thesis is directed at unveiling properties of AGN structure through broadband X-ray spectroscopy, with particular emphasis on the hard X-ray band (photon energies above 10 keV). With its unprecedented sensitivity in this energy band, the NuSTAR telescope provides the key observational diagnostics of the properties of the AGN X-ray source, the corona, and the surrounding gas in the accretion disk, the broad-line region, and the torus.
\r\n\r\nThe first study presented in this thesis focuses on measurements of the optical depth and the temperature of the plasma in the corona of an obscured AGN. Fitting theoretical spectral models for coronal emission to the NuSTAR data constrained these two basic physical parameters under the assumption of either spherical or disk-like geometry for the corona.
\r\n\r\n\r\nThe remainder of the thesis is dedicated to studies of the anisotropic obscuring structure broadly referred to as the torus. One of them is a case study of three heavily obscured AGN with spectra dominated by the X-ray light scattered and reprocessed in the torus, where it is possible to constrain one of the basic torus properties -- its globally averaged column density. The following study presents the calculation of a new spectral model for reprocessing of the intrinsic X-ray continuum within the torus. Its added flexibility compared to previously available models allows for both the average column density of the torus and its covering factor to be constrained from broadband X-ray spectra of a wide variety of AGN.
\r\n\r\n\r\nThe final part of the thesis in based on a large survey of the local obscured AGN population performed with NuSTAR. Spectral modeling of more than a hundred individual AGN, including both old and new spectral models, is presented. From analyses of the X-ray data for a large and representative AGN sample, for the first time, it is found that their tori preferentially have high covering factors and average column densities close to unity optical depth for Compton scattering.
\r\n", "doi": "10.7907/Z9WM1BG8", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:9906", "collection": "thesis", "collection_id": "9906", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08292016-174647631", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 8434177, "license": "other", "mime_type": "application/pdf", "url": "/9906/1/thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Low-Mass Stars and Their Companions", "author": [ { "family_name": "Montet", "given_name": "Benjamin Tyler", "orcid": "0000-0001-7516-8308", "clpid": "Montet-Benjamin-Tyler" } ], "thesis_advisor": [ { "family_name": "Johnson", "given_name": "John A.", "clpid": "Johnson-J-A" } ], "thesis_committee": [ { "family_name": "Hallinan", "given_name": "Gregg W.", "orcid": "0000-0002-7083-4049", "clpid": "Hallinan-G-W" }, { "family_name": "Johnson", "given_name": "John A.", "clpid": "Johnson-J-A" }, { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" }, { "family_name": "Readhead", "given_name": "Anthony C. S.", "orcid": "0000-0001-9152-961X", "clpid": "Readhead-A-C-S" }, { "family_name": "Knutson", "given_name": "Heather A.", "clpid": "Knutson-H-A" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "In this thesis, I present seven studies aimed towards better understanding the demographics and physical properties of M dwarfs and their companions. These studies focus in turn on planetary, brown dwarf, and stellar companions to M dwarfs.
\r\n\r\nI begin with an analysis of radial velocity and transit timing analyses of multi-transiting planetary systems, finding that if both signals are measured to sufficiently high precision the stellar and planetary masses can be measured to a high precision, eliminating a need for stellar models which may have systematic errors. I then combine long-term radial velocity monitoring and a direct imaging campaign to measure the occurrence rate of giant planets around M dwarfs. I find that 6.5 +/- 3.0% of M dwarfs host a Jupiter mass or larger planet within 20 AU, with a strong dependence on stellar metallicity.
\r\n\r\nI then present two papers analyzing the LHS 6343 system, which contains a widely separated M dwarf binary (AB). Star A hosts a transiting brown dwarf (LHS 6343 C) with a 12.7 day period. By combining radial velocity data with transit photometry, I am able to measure the mass and radius of the brown dwarf to 2% precision, the most precise measurement of a brown dwarf to date. I then analyze four secondary eclipses of the LHS 6343 AC system as observed by Spitzer in order to measure the luminosity of the brown dwarf in both Spitzer bandpasses. I find the brown dwarf is consistent with theoretical models of an 1100 K T dwarf at an age of 5 Gyr and empirical observations of field T5-6 dwarfs with temperatures of 1070 +/- 130 K. This is the first non-inflated brown dwarf with a measured mass, radius, and multi-band photometry, making it an ideal test of evolutionary models of field brown dwarfs.
\r\n\r\nNext, I present the results of an astrometric and radial velocity campaign to measure the orbit and masses of both stars in the GJ 3305 AB system, an M+M binary comoving with 51 Eridani, a more massive star with a directly imaged planetary companion. I compare the masses of both stars to largely untested theoretical models of young M dwarfs, finding that the models are consistent with the measured mass of star A but slightly overpredict the luminosity of star B.
\r\n\r\nIn the final two science chapters I focus on space-based transit surveys, present and future. First, I present the first catalog of statistically validated planets from the K2 mission, as well as updated stellar and planetary parameters for all systems with candidate planets in the first K2 field. The catalog includes K2-18b, a ``mini-Neptune'' planet that receives a stellar insolation consistent with the level that the Earth receives from the Sun, making it a useful comparison against planets of a similar size that are highly irradiated, such as GJ 1214 b. Finally, I present predictions for the WFIRST mission. While designed largely as a microlensing mission, I find it will be able to detect as many as 30,000 transiting planets towards the galactic bulge, providing information about how planet occurrence changes across the galaxy. These planets will be able to be confirmed largely through direct detection of their secondary eclipses. Moreover, I find that more than 50% of the planets it detects smaller than Neptune will be found around M dwarf hosts.
", "doi": "10.7907/Z9KW5D1R", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:9604", "collection": "thesis", "collection_id": "9604", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03042016-144127692", "primary_object_url": { "basename": "Swarnima-Manohar-thesis-2016.pdf", "content": "final", "filesize": 8264476, "license": "other", "mime_type": "application/pdf", "url": "/9604/1/Swarnima-Manohar-thesis-2016.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Lurking in ULIRGs: Molecular Gas in local Merging Galaxies", "author": [ { "family_name": "Manohar", "given_name": "Swarnima", "orcid": "0000-0002-3506-566X", "clpid": "Manohar-Swarnima" } ], "thesis_advisor": [ { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "orcid": "0000-0002-0438-3323", "clpid": "Scoville-N-Z" } ], "thesis_committee": [ { "family_name": "Sargent", "given_name": "Anneila Isabel", "orcid": "0000-0002-4633-5098", "clpid": "Sargent-A-I" }, { "family_name": "Readhead", "given_name": "Anthony C. S.", "orcid": "0000-0001-9152-961X", "clpid": "Readhead-A-C-S" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "orcid": "0000-0002-0438-3323", "clpid": "Scoville-N-Z" } ], "local_group": [ { "literal": "Owens Valley Radio Observatory (OVRO)" }, { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "Mergers and interacting galaxies are pivotal to the evolution of galaxies in the universe. They are the sites of prodigious star formation and key to understanding the starburst processes: the physical and chemical properties and the dynamics of the molecular gas. ULIRGs or Ultraluminous Infrared Galaxies are a result of many of these mergers. They host extreme starbursts, AGNs, and mergers. They are the perfect laboratory to probe the connection between starbursts, black hole accretion and mergers and to further our understanding of star formation and merging.
\r\n\r\nNGC 6240 and Arp 220 can be considered the founding members of this very active class of objects. They are in different stages of merging and hence are excellent case studies to further our understanding about the merging process. We have imaged the dense star-forming regions of these galaxies at sub-arcsec resolution with CARMA C and B Configurations (2\" and 0.5 - 0.8\"). Multi-band imaging allows excitation analysis of HCN, HCO+, HNC, and CS along with CO transitions to constrain the properties of the gas. Our dataset is unique in that we have observed these lines at similar resolutions and high sensitivity which can be used to derive line ratios of faint high excitation lines.
\r\n\r\nArp 220 has not had confirmed X-ray AGN detections for either nuclei. However, our observations indicate HCN/HNC ratios consistent with the chemistry of X-ray Dominated Regions (XDRs) -- a likely symptom of AGN. We calculated the molecular Hydrogen densities using each of the molecular species and conclude that assuming abundances of HNC and HCO+ similar to those in galactic sources are incorrect in the case of ULIRGs. The physical conditions in the dense molecular gas in ULIRGs alter these abundances. The derived H2 volume densities are ~ 5 x 104 cm-3 in both Arp 220 nuclei and ~ 104 cm-3 in NGC 6240.
", "doi": "10.7907/Z9T72FDK", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9238", "collection": "thesis", "collection_id": "9238", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10212015-211417853", "primary_object_url": { "basename": "siegel_seth_2016_thesis.pdf", "content": "final", "filesize": 29094572, "license": "other", "mime_type": "application/pdf", "url": "/9238/1/siegel_seth_2016_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Multiwavelength Study of the Intracluster Medium and the Characterization of the Multiwavelength Sub/millimeter Inductance Camera", "author": [ { "family_name": "Siegel", "given_name": "Seth Robert", "orcid": "0000-0003-2631-6217", "clpid": "Siegel-Seth-Robert" } ], "thesis_advisor": [ { "family_name": "Golwala", "given_name": "Sunil", "orcid": "0000-0002-1098-7174", "clpid": "Golwala-S-R" } ], "thesis_committee": [ { "family_name": "Golwala", "given_name": "Sunil", "orcid": "0000-0002-1098-7174", "clpid": "Golwala-S-R" }, { "family_name": "Zmuidzinas", "given_name": "Jonas", "orcid": "0000-0002-3330-5439", "clpid": "Zmuidzinas-J" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Bartlett", "given_name": "James", "clpid": "Bartlett-James" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "The first part of this thesis combines Bolocam observations of the thermal Sunyaev-Zel\u2019dovich (SZ) effect at 140 GHz with X-ray observations from Chandra, strong lensing data from the Hubble Space Telescope (HST), and weak lensing data from HST and Subaru to constrain parametric models for the distribution of dark and baryonic matter in a sample of six massive, dynamically relaxed galaxy clusters. For five of the six clusters, the full multiwavelength dataset is well described by a relatively simple model that assumes spherical symmetry, hydrostatic equilibrium, and entirely thermal pressure support. The multiwavelength analysis yields considerably better constraints on the total mass and concentration compared to analysis of any one dataset individually. The subsample of five galaxy clusters is used to place an upper limit on the fraction of pressure support in the intracluster medium (ICM) due to nonthermal processes, such as turbulent and bulk flow of the gas. We constrain the nonthermal pressure fraction at r500c to be less than 0.11 at 95% confidence, where r500c refers to radius at which the average enclosed density is 500 times the critical density of the Universe. This is in tension with state-of-the-art hydrodynamical simulations, which predict a nonthermal pressure fraction of approximately 0.25 at r500c for the clusters in this sample.
\r\n\r\nThe second part of this thesis focuses on the characterization of the Multiwavelength Sub/millimeter Inductance Camera (MUSIC), a photometric imaging camera that was commissioned at the Caltech Submillimeter Observatory (CSO) in 2012. MUSIC is designed to have a 14 arcminute, diffraction-limited field of view populated with 576 spatial pixels that are simultaneously sensitive to four bands at 150, 220, 290, and 350 GHz. It is well-suited for studies of dusty star forming galaxies, galaxy clusters via the SZ Effect, and galactic star formation. MUSIC employs a number of novel detector technologies: broadband phased-arrays of slot dipole antennas for beam formation, on-chip lumped element filters for band definition, and Microwave Kinetic Inductance Detectors (MKIDs) for transduction of incoming light to electric signal. MKIDs are superconducting micro-resonators coupled to a feedline. Incoming light breaks apart Cooper pairs in the superconductor, causing a change in the quality factor and frequency of the resonator. This is read out as amplitude and phase modulation of a microwave probe signal centered on the resonant frequency. By tuning each resonator to a slightly different frequency and sending out a superposition of probe signals, hundreds of detectors can be read out on a single feedline. This natural capability for large scale, frequency domain multiplexing combined with relatively simple fabrication makes MKIDs a promising low temperature detector for future kilopixel sub/millimeter instruments. There is also considerable interest in using MKIDs for optical through near-infrared spectrophotometry due to their fast microsecond response time and modest energy resolution. In order to optimize the MKID design to obtain suitable performance for any particular application, it is critical to have a well-understood physical model for the detectors and the sources of noise to which they are susceptible. MUSIC has collected many hours of on-sky data with over 1000 MKIDs. This work studies the performance of the detectors in the context of one such physical model. Chapter 2 describes the theoretical model for the responsivity and noise of MKIDs. Chapter 3 outlines the set of measurements used to calibrate this model for the MUSIC detectors. Chapter 4 presents the resulting estimates of the spectral response, optical efficiency, and on-sky loading. The measured detector response to Uranus is compared to the calibrated model prediction in order to determine how well the model describes the propagation of signal through the full instrument. Chapter 5 examines the noise present in the detector timestreams during recent science observations. Noise due to fluctuations in atmospheric emission dominate at long timescales (less than 0.5 Hz). Fluctuations in the amplitude and phase of the microwave probe signal due to the readout electronics contribute significant 1/f and drift-type noise at shorter timescales. The atmospheric noise is removed by creating a template for the fluctuations in atmospheric emission from weighted averages of the detector timestreams. The electronics noise is removed by using probe signals centered off-resonance to construct templates for the amplitude and phase fluctuations. The algorithms that perform the atmospheric and electronic noise removal are described. After removal, we find good agreement between the observed residual noise and our expectation for intrinsic detector noise over a significant fraction of the signal bandwidth.
", "doi": "10.7907/Z9Z31WJ7", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9243", "collection": "thesis", "collection_id": "9243", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10232015-070212778", "type": "thesis", "title": "Deep Near-Infrared Spectroscopy of High-Redshift Galaxies: the Physical Growth of Passive Systems", "author": [ { "family_name": "Belli", "given_name": "Sirio", "orcid": "0000-0002-5615-6018", "clpid": "Belli-Sirio" } ], "thesis_advisor": [ { "family_name": "Ellis", "given_name": "Richard S.", "clpid": "Ellis-R-S" } ], "thesis_committee": [ { "family_name": "Readhead", "given_name": "Anthony C. S.", "orcid": "0000-0001-9152-961X", "clpid": "Readhead-A-C-S" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Hallinan", "given_name": "Gregg W.", "clpid": "Hallinan-G-W" }, { "family_name": "Ellis", "given_name": "Richard S.", "clpid": "Ellis-R-S" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "The assembly history of massive galaxies is one of the most important aspects of galaxy formation and evolution. Although we have a broad idea of what physical processes govern the early phases of galaxy evolution, there are still many open questions. In this thesis I demonstrate the crucial role that spectroscopy can play in a physical understanding of galaxy evolution. I present deep near-infrared spectroscopy for a sample of high-redshift galaxies, from which I derive important physical properties and their evolution with cosmic time. I take advantage of the recent arrival of efficient near-infrared detectors to target the rest-frame optical spectra of z > 1 galaxies, from which many physical quantities can be derived. After illustrating the applications of near-infrared deep spectroscopy with a study of star-forming galaxies, I focus on the evolution of massive quiescent systems.
\r\n\r\nMost of this thesis is based on two samples collected at the W. M. Keck Observatory that represent a significant step forward in the spectroscopic study of z > 1 quiescent galaxies. All previous spectroscopic samples at this redshift were either limited to a few objects, or much shallower in terms of depth. Our first sample is composed of 56 quiescent galaxies at 1 < z < 1.6 collected using the upgraded red arm of the Low Resolution Imaging Spectrometer (LRIS). The second consists of 24 deep spectra of 1.5 < z < 2.5 quiescent objects observed with the Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE). Together, these spectra span the critical epoch 1 < z < 2.5, where most of the red sequence is formed, and where the sizes of quiescent systems are observed to increase significantly.
\r\n\r\nWe measure stellar velocity dispersions and dynamical masses for the largest number of z > 1 quiescent galaxies to date. By assuming that the velocity dispersion of a massive galaxy does not change throughout its lifetime, as suggested by theoretical studies, we match galaxies in the local universe with their high-redshift progenitors. This allows us to derive the physical growth in mass and size experienced by individual systems, which represents a substantial advance over photometric inferences based on the overall galaxy population. We find a significant physical growth among quiescent galaxies over 0 < z < 2.5 and, by comparing the slope of growth in the mass-size plane dlogRe/dlogM\u2217 with the results of numerical simulations, we can constrain the physical process responsible for the evolution. Our results show that the slope of growth becomes steeper at higher redshifts, yet is broadly consistent with minor mergers being the main process by which individual objects evolve in mass and size.
\r\n\r\nBy fitting stellar population models to the observed spectroscopy and photometry we derive reliable ages and other stellar population properties. We show that the addition of the spectroscopic data helps break the degeneracy between age and dust extinction, and yields significantly more robust results compared to fitting models to the photometry alone. We detect a clear relation between size and age, where larger galaxies are younger. Therefore, over time the average size of the quiescent population will increase because of the contribution of large galaxies recently arrived to the red sequence. This effect, called progenitor bias, is different from the physical size growth discussed above, but represents another contribution to the observed difference between the typical sizes of low- and high-redshift quiescent galaxies. By reconstructing the evolution of the red sequence starting at z \u223c 1.25 and using our stellar population histories to infer the past behavior to z \u223c 2, we demonstrate that progenitor bias accounts for only half of the observed growth of the population. The remaining size evolution must be due to physical growth of individual systems, in agreement with our dynamical study.
\r\n\r\nFinally, we use the stellar population properties to explore the earliest periods which led to the formation of massive quiescent galaxies. We find tentative evidence for two channels of star formation quenching, which suggests the existence of two independent physical mechanisms. We also detect a mass downsizing, where more massive galaxies form at higher redshift, and then evolve passively. By analyzing in depth the star formation history of the brightest object at z > 2 in our sample, we are able to put constraints on the quenching timescale and on the properties of its progenitor.
\r\n\r\nA consistent picture emerges from our analyses: massive galaxies form at very early epochs, are quenched on short timescales, and then evolve passively. The evolution is passive in the sense that no new stars are formed, but significant mass and size growth is achieved by accreting smaller, gas-poor systems. At the same time the population of quiescent galaxies grows in number due to the quenching of larger star-forming galaxies. This picture is in agreement with other observational studies, such as measurements of the merger rate and analyses of galaxy evolution at fixed number density.
", "doi": "10.7907/Z9QN64NQ ", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:8636", "collection": "thesis", "collection_id": "8636", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08182014-123907723", "primary_object_url": { "basename": "trainor_ryan_thesis_full_2015.pdf", "content": "final", "filesize": 2782421, "license": "other", "mime_type": "application/pdf", "url": "/8636/14/trainor_ryan_thesis_full_2015.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Faint Galaxies in the Megaparsec-Scale Environments of Hyperluminous QSOs at Redshifts 2 < z < 3", "author": [ { "family_name": "Trainor", "given_name": "Ryan Francis", "clpid": "Trainor-Ryan-Francis" } ], "thesis_advisor": [ { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" } ], "thesis_committee": [ { "family_name": "Hillenbrand", "given_name": "Lynne A.", "clpid": "Hillenbrand-L-A" }, { "family_name": "Steidel", "given_name": "Charles C.", "orcid": "0000-0002-4834-7260", "clpid": "Steidel-C-C" }, { "family_name": "Martin", "given_name": "D. Christopher", "orcid": "0000-0002-8650-1644", "clpid": "Martin-D-Christopher" }, { "family_name": "Hopkins", "given_name": "Philip F.", "orcid": "0000-0003-3729-1684", "clpid": "Hopkins-P-F" }, { "family_name": "Kollmeier", "given_name": "Juna A.", "orcid": "0000-0001-9852-1610", "clpid": "Kollmeier-J-A" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "This thesis presents detailed observational studies of the extended distributions of gas, galaxies, and dark matter around hyperluminous quasars (HLQSOs) at high redshift. Taken together, these works aim to coherently describe the relationships between these massive, accreting black holes and their environments: the nature of the regions that give rise to such massive black holes, the effect of HLQSO radiation on their surrounding galaxies and gas, and the ability of both galaxies and black holes to shed new light on the formation and evolution of the other.
\r\n\r\nChapter 2 focuses on the continuum-color-selected galaxies drawn from the Keck Baryonic Structure Survey (KBSS). The KBSS is a uniquely deep spectroscopic survey of star-forming galaxies in the same volumes of space as 15 HLQSOs at 2.5 <
Chapter 3 describes the first results from a new survey (KBSS-Lyα) conducted for this thesis. The KBSS-Lyα survey uses narrowband imaging to identify Lyα-emitters (LAEs) in the ~Mpc regions around eight of the KBSS HLQSOs. Many of these LAEs show the effect of reprocessed HLQSO radiation in their emission through the process known as Lyα fluorescence. In this chapter, these fluorescent LAEs are used to generate a coarse map of the average HLQSO ionizing emission on Mpc scales, thereby setting the first direct constraints of the lifetime and angular distribution of activity for a population of these uniquely luminous black holes.
\r\n\r\nChapter 4 contains a more detailed description of the KBSS-Lyα survey itself and the detailed properties of the star-forming and fluorescent objects selected therein. Using imaging and spectroscopic data covering rest-frame UV and optical wavelengths, including spectra from the new near-infrared spectrometer MOSFIRE, we characterize this population of nascent galaxies in terms of their kinematics, enrichment, gas properties, and luminosity distribution while comparing and contrasting them with previously-studied populations of continuum-selected galaxies and LAEs far from the effects of HLQSO emission.
\r\n\r\nAt the conclusion of this thesis, I briefly present future directions for the continuation of this research. In Appendix A, I provide background information on the instrumentation used in this thesis, including my own contributions to MOSFIRE.
", "doi": "10.7907/Z90K26JG", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8626", "collection": "thesis", "collection_id": "8626", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08012014-181643065", "primary_object_url": { "basename": "Schenker_June_18_2014.pdf", "content": "final", "filesize": 2808296, "license": "other", "mime_type": "application/pdf", "url": "/8626/1/Schenker_June_18_2014.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Did Galaxies Reionize the Universe?", "author": [ { "family_name": "Schenker", "given_name": "Matthew Aaron", "clpid": "Schenker-Matthew-Aaron" } ], "thesis_advisor": [ { "family_name": "Ellis", "given_name": "Richard S.", "clpid": "Ellis-R-S" } ], "thesis_committee": [ { "family_name": "Hallinan", "given_name": "Gregg W.", "clpid": "Hallinan-G-W" }, { "family_name": "Steidel", "given_name": "Charles C.", "clpid": "Steidel-C-C" }, { "family_name": "Scoville", "given_name": "Nicholas Zabriskie", "clpid": "Scoville-N-Z" }, { "family_name": "Hopkins", "given_name": "Philip F.", "clpid": "Hopkins-P-F" }, { "family_name": "Ellis", "given_name": "Richard S.", "clpid": "Ellis-R-S" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "The epoch of reionization remains one of the last uncharted eras of cosmic history, yet this time is of crucial importance, encompassing the formation of both the first galaxies and the first metals in the universe. In this thesis, I present four related projects that both characterize the abundance and properties of these first galaxies and uses follow-up observations of these galaxies to achieve one of the first observations of the neutral fraction of the intergalactic medium during the heart of the reionization era.
\r\n\r\nFirst, we present the results of a spectroscopic survey using the Keck telescopes targeting 6.3 < z < 8.8 star-forming galaxies. We secured observations of 19 candidates, initially selected by applying the Lyman\r\nbreak technique to infrared imaging data from the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope (HST). This survey builds upon earlier work from Stark et al. (2010, 2011), which showed that star-forming galaxies at 3 < z < 6, when the universe was highly ionized, displayed a significant increase in strong Lyman alpha emission with redshift. Our work uses the LRIS and NIRSPEC instruments to search for Lyman alpha emission in candidates at a greater redshift in the observed near-infrared, in order to discern if this evolution continues, or is quenched by an increase in the neutral fraction of the intergalactic medium. Our spectroscopic observations\r\ntypically reach a 5-sigma limiting sensitivity of < 50 AA. Despite expecting to detect Lyman alpha at 5-sigma in 7-8 galaxies based on our Monte Carlo simulations, we only achieve secure detections in two of 19 sources. Combining these results with a similar sample of 7 galaxies from Fontana et al. (2010), we determine that these few detections would only occur in < 1% of simulations if the intrinsic distribution was the same as that at z ~ 6. We consider other explanations for this decline, but find the most convincing explanation to be an increase in the neutral fraction of the intergalactic medium. Using theoretical models, we infer a neutral fraction of X_HI ~ 0.44 at z = 7.
\r\n\r\nSecond, we characterize the abundance of star-forming galaxies at z > 6.5 again using WFC3 onboard the HST. This project conducted a detailed search for candidates both in the Hubble Ultra Deep Field as well as a number of additional wider Hubble Space Telescope surveys to construct luminosity\r\nfunctions at both z ~ 7 and 8, reaching 0.65 and 0.25 mag fainter than any previous surveys, respectively. With this increased depth, we achieve some of the most robust constraints on the Schechter function faint end slopes\r\nat these redshifts, finding very steep values of alpha_{z~7} = -1.87 +/- 0.18 and alpha_{z~8} = -1.94 +/- 0.23. We discuss these results in the context of cosmic reionization, and show that given reasonable assumptions about the ionizing spectra and escape fraction of ionizing photons, only half the photons needed to maintain reionization are provided by currently observable galaxies at z ~ 7-8. We show that an extension of the luminosity function down to M_{UV} = -13.0, coupled with a low level of star-formation out to\r\nhigher redshift, can fit all available constraints on the ionization history of the universe.
\r\n\r\nThird, we investigate the strength of nebular emission in 3 < z < 5 star-forming galaxies. We begin by using the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope to investigate the strength of \r\nH alpha emission in a sample of 3.8 < z < 5.0 spectroscopically confirmed galaxies. We then conduct near-infrared observations of star-forming galaxies at 3 < z < 3.8 to investigate the strength of the [OIII] 4959/5007 and H beta emission lines from the ground using MOSFIRE. In both cases, we uncover near-ubiquitous strong nebular emission, and find excellent agreement between the fluxes derived using the separate methods. For a subset of 9 objects in our MOSFIRE sample\r\nthat have secure Spitzer IRAC detections, we compare the emission\r\nline flux derived from the excess in the K_s band photometry to that derived from direct spectroscopy and find 7 to agree within a factor of 1.6, with only one catastrophic outlier. Finally, for a different subset for which we also\r\nhave DEIMOS rest-UV spectroscopy, we compare the relative velocities of Lyman alpha and the rest-optical nebular lines which should trace the cites of star-formation. We find a median velocity offset of only \r\nv_{Ly alpha} = 149 km/s, significantly less than the 400 km/s observed for star-forming galaxies with weaker Lyman alpha emission at z = 2-3 (Steidel et al. 2010), and show that this decrease can be explained by a decrease \r\nin the neutral hydrogen column density covering the galaxy. We discuss how this will imply a lower neutral fraction for a given observed extinction of Lyman alpha when its visibility is used to probe the ionization state of the intergalactic medium.
\r\n\r\nFinally, we utilize the recent CANDELS wide-field, infra-red photometry over the GOODS-N and S fields to re-analyze the use of Lyman alpha emission to evaluate the neutrality of the intergalactic medium. With this new data, we derive accurate ultraviolet spectral slopes for a sample of 468 3 < z < 6 star-forming galaxies, already observed in the rest-UV with the Keck spectroscopic survey (Stark et al. 2010). We use a Bayesian fitting method which accurately\r\naccounts for contamination and obscuration by skylines to derive a relationship between the UV-slope of a galaxy and its intrinsic Lyman alpha equivalent width probability distribution. We then apply this data to spectroscopic surveys during the reionization era, including our own, to accurately interpret the drop in observed Lyman alpha emission. From our most recent such MOSFIRE survey, we also present evidence for the most distant galaxy confirmed through emission line spectroscopy at z = 7.62, as well as a first detection of the CIII]1907/1909 doublet at z > 7.
\r\n\r\nWe conclude the thesis by exploring future prospects and summarizing the results of Robertson et al. (2013). This work synthesizes many of the measurements in this thesis, along with external constraints, to create a model of reionization that fits nearly all available constraints.
", "doi": "10.7907/Z9736NVR", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" } ]