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: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: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: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: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: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: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: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: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: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" } ]