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:17159", "collection": "thesis", "collection_id": "17159", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04162025-220303394", "type": "thesis", "title": "Numerical Modeling of High-energy Transients from Black Holes and Neutron Stars", "author": [ { "family_name": "Kim", "given_name": "Yoonsoo", "orcid": "0000-0002-4305-602", "clpid": "Kim-Yoonsoo" } ], "thesis_advisor": [ { "family_name": "Most", "given_name": "Elias R.", "orcid": "0000-0002-0491-1210", "clpid": "Most-E-R" }, { "family_name": "Teukolsky", "given_name": "Saul A.", "orcid": "0000-0001-9765-4526", "clpid": "Teukolsky-S-A" } ], "thesis_committee": [ { "family_name": "Teukolsky", "given_name": "Saul A.", "orcid": "0000-0001-9765-4526", "clpid": "Teukolsky-S-A" }, { "family_name": "Phinney", "given_name": "E. Sterl", "orcid": "0000-0002-9656-4032", "clpid": "Phinney-E-S" }, { "family_name": "Graham", "given_name": "Matthew J.", "orcid": "0000-0002-3168-0139", "clpid": "Graham-M-J" }, { "family_name": "Most", "given_name": "Elias R.", "orcid": "0000-0002-0491-1210", "clpid": "Most-E-R" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Along with recent breakthroughs in relativistic astrophysics and multi-messenger astronomy, theoretical studies on compact objects and the dynamics of relativistic matter surrounding them have a growing significance. General relativistic approaches are required to properly describe astrophysical phenomena taking place in a strong gravity regime, yet the high complexity and nonlinearity of the equations governing those systems compel numerical approaches. In this thesis, we develop a computational method for and present global numerical simulations of relativistic plasma around compact objects, particularly focusing on high-energy electromagnetic transients originating from black holes and neutron stars. Our works include a new hybrid numerical scheme for modeling force-free magnetospheres of compact objects, large-scale simulations of a spinning black hole immersed in a magnetized wind, and magnetospheric transients from a merging black hole--neutron star binary.", "doi": "10.7907/8skm-3x17", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17372", "collection": "thesis", "collection_id": "17372", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022025-193721130", "primary_object_url": { "basename": "habib_sarah_2025.pdf", "content": "final", "filesize": 1447710, "license": "other", "mime_type": "application/pdf", "url": "/17372/1/habib_sarah_2025.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Error Quantification and Mitigation for Numerical Compact Binary Waveforms", "author": [ { "family_name": "Habib", "given_name": "Sarah Mary", "orcid": "0000-0002-4725-4978", "clpid": "Habib-Sarah-Mary" } ], "thesis_advisor": [ { "family_name": "Teukolsky", "given_name": "Saul A.", "orcid": "0000-0001-9765-4526", "clpid": "Teukolsky-S-A" }, { "family_name": "Scheel", "given_name": "Mark", "orcid": "0000-0001-6656-9134", "clpid": "Scheel-M-A" }, { "family_name": "Most", "given_name": "Elias R.", "orcid": "0000-0002-0491-1210", "clpid": "Most-E-R" } ], "thesis_committee": [ { "family_name": "Most", "given_name": "Elias R.", "orcid": "0000-0002-0491-1210", "clpid": "Most-E-R" }, { "family_name": "Teukolsky", "given_name": "Saul A.", "orcid": "0000-0001-9765-4526", "clpid": "Teukolsky-S-A" }, { "family_name": "Scheel", "given_name": "Mark", "orcid": "0000-0001-6656-9134", "clpid": "Scheel-M-A" }, { "family_name": "Weinstein", "given_name": "Alan Jay", "orcid": "0000-0002-0928-6784", "clpid": "Weinstein-Alan-J-Physics" }, { "family_name": "Chatziioannou", "given_name": "Katerina", "orcid": "0000-0002-5833-413X", "clpid": "Chatziioannou-K" }, { "family_name": "Chen", "given_name": "Yanbei", "orcid": "0000-0002-9730-9463", "clpid": "Chen-Yanbei" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Gravitational wave analysis requires waveform models to compare with observed signals from compact binaries. These models are based on and validated by numerical relativity waveforms---waveforms output from codes developed to numerically evolve the Einstein field equations. The efficacy of numerical waveforms for analysis is limited by error from both numerical and astrophysical sources. This thesis makes two contributions to the quantification and mitigation of this error.
\r\n\r\nChapter 2 describes a new algorithm for eccentricity reduction, the process of determining initial conditions for quasicircular binary orbits. This iterative procedure requires a measurement of eccentricity based on an early-inspiral trajectory. We find that the use of nonlinear fitting techniques such as variable projection leads to vastly improved consistency in eccentricity measurements.
\r\n\r\nFinally, Chapter 3 presents an in-depth quantification of error in numerical binary neutron star waveforms from three vastly different numerical relativity codes. We find that overall these codes produce consistent binary neutron star evolutions, but that further accuracy improvements will be required for analysis of next-generation gravitational wave detector signals.
", "doi": "10.7907/jbre-7e68", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17280", "collection": "thesis", "collection_id": "17280", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282025-025356461", "primary_object_url": { "basename": "Rui_Nicholas_2025_web-friendly.pdf", "content": "final", "filesize": 15933062, "license": "other", "mime_type": "application/pdf", "url": "/17280/1/Rui_Nicholas_2025_web-friendly.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Seismic Probes of Stellar Mergers and Magnetism", "author": [ { "family_name": "Rui", "given_name": "Nicholas Zhao", "orcid": "0000-0002-1884-3992", "clpid": "Rui-Nicholas-Zhao" } ], "thesis_advisor": [ { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" } ], "thesis_committee": [ { "family_name": "Phinney", "given_name": "E. Sterl", "orcid": "0000-0002-9656-4032", "clpid": "Phinney-E-S" }, { "family_name": "Fuller", "given_name": "James", "orcid": "0000-0002-4544-0750", "clpid": "Fuller-J" }, { "family_name": "El-Badry", "given_name": "Kareem J.", "orcid": "0000-0002-6871-1752", "clpid": "El-Badry-K-J" }, { "family_name": "Kasliwal", "given_name": "Mansi M.", "orcid": "0000-0002-5619-4938", "clpid": "Kasliwal-M-M" }, { "family_name": "Most", "given_name": "Elias R.", "orcid": "0000-0002-0491-1210", "clpid": "Most-E-R" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Stellar pulsations can do what most other astrophysical observables cannot: directly probe internal stellar properties. This thesis consolidates work investigating how stellar oscillation modes are affected by two common but \"noncanonical\" pieces of stellar physics: mergers and magnetism.
\r\n\r\nThe earlier chapters develop \"seismic stellar merger genealogy,\" the application of seismology to the discovery of stellar merger remnants. In Chapter II, I show that red giants which have engulfed close, main-sequence companions possess unusual gravity-mode period spacings, indicating their binary origin. I identify two dozen promising merger remnant candidates in archival Kepler data, roughly consistent with expected stellar merger rates. In Chapter III, I study the evolution and properties of the red-giant-like stars which result from coalescences of accreting helium-core white dwarf systems. These merger remnants display distinctive seismic and chemical properties, particularly during the core helium-burning phase as the result of an especially violent helium flash.
\r\n\r\nThe later chapters develop \"seismic stellar magnetometry,\" the application of seismology to the measurement of stellar magnetic fields. In Chapter IV, I calculate the morphology of high-radial-order gravity modes under the influence of strong magnetic fields. The eigenfunctions exhibit two morphological features at which energy dissipation may be strong, in agreement with the suppressed dipole modes observed in many red giants.\r\nIn Chapter V, I apply the same method to calculate the gravity-mode period spacing pattern under a strong magnetic field. The perturbative theory developed for weak fields underestimates the true frequency shifts to gravity modes caused by strong magnetic fields. In Chapter VI, I model the behavior of stochastic pulsators whose magnetic fields are strong enough to misalign their pulsations from the rotation axis. Even in the presence of stochasticity, the light curves of such oblique pulsators indefinitely retain some phase information in a way that can be used to identify them. In Chapter VII, I place upper bounds on the near-surface magnetic fields of a sample of white dwarfs based on the non-detection of magnetic features in their pulsation spectra. Although these constraints vary significantly with white dwarf structure and mode periods, they are consistently much stronger than the megagauss-scale magnetic fields to which spectroscopy is sensitive.
", "doi": "10.7907/9stk-9462", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" } ]