Astronomy
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Item Theoretical, Experimental, and Observational Studies of Iron X-ray Spectra: From the Laboratory to the Universe(2024) Grell, Gabriel Jonathan; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The spectral lines of iron ions, particularly the dominant neon-like Fe XVII charge state, provide crucial diagnostics for the physical conditions of hot astrophysical plasmas in the X-ray regime. However, the diagnostic utility of these lines are hampered by significant discrepancies at the ~20% level between spectral observations, laboratory experiments, and theoretical calculations of the astrophysically important Fe XVII transitions, an issue that has been observed in numerous studies over several decades. Understanding the source of these discrepancies is critical for the improvement of both theoretical atomic models and laboratory experiment data on transition energies and cross sections of electron-ion processes, which themselves will be key for comparison to observations from X-ray spectroscopy missions such as XRISM, Line Emission Mapper (LEM), Arcus, and Athena. My dissertation encapsulates the main branches of X-ray astrophysics by focusing on the use of theoretical models and experimental measurements to further the diagnostic use, understanding, and interpretation of spectroscopic observations of iron transition lines. I modeled the effects of UV photoexcitation in O-type stars on a spectral line ratio of the Fe XVII 3s – 2p transitions in an attempt to explain an anomalous value found for the X-ray spectra of the O star ζ Puppis. I conjectured that the strong UV field of ζ Pup produces the observed ratio by depopulation of metastable 3s excited states, and that the ratio can potentially be used as an independent diagnostic of the radial distribution of X-ray-emitting plasma. Using the Flexible Atomic Code (FAC) collisional-radiative model to model the effect of UV photoexcitation on the Fe XVII lines, I compared the model calculations to archival spectra of coronal and hot stars from the Chandra HETGS and XMM-Newton RGS. The calculations showed that UV photoexcitation does not produce a sufficiently large dynamic range in the Fe XVII line ratio to explain the difference in the observed ratio between coronal stars and ζ Pup. I used FAC to compute steady-state populations of Fe XVII states and calculate cross sections for the dielectronic recombination (DR) and direct electron-impact excitation (DE) line formation channels of Fe XVII, and benchmarked the model predictions with experimental cross sections of Fe XVII resonances that were mono-energetically excited in an electron beam ion trap (EBIT) experiment. I extended the benchmark to all resolved DR and DE channels in the experimental dataset with a focus on the n ≥ 4 DR resonances, finding that the DR and DE absolute cross section predictions for the higher n complexes disagree considerably with experimental results when using the same methods as in previous works. However, agreement within ∼10% of the experimental results was achieved by an approach whereby I doubly convolve the predicted cross sections with both the spread of the electron-beam energy and the photon-energy resolution of the EBIT experiment. I also calculated rate coefficients from the experimental and theoretical cross sections, finding general agreement within 2σ with the rates found in the OPEN-ADAS atomic database. Circling back to the ζ Pup Fe XVII ratio, I probed the potential significance of the process of resonant Auger destruction (RAD), which occurs when a photon emitted by an ion is absorbed in a neighboring cooler part of the stellar wind by near-coincident inner-shell transitions of lower charge state ions. The inner-shell excited ion then undergoes Auger decay, in which the energy is transferred to an outer electron that is subsequently ejected from the atom by autoionization. EBIT measurements at a synchrotron beamline determined that 3d – 2p transitions of the lower iron charge state Fe VI is nearly coincident in transition energy with the Fe XVII 3G line, which would enable possible destruction of Fe XVII 3G photons and thus a potential explanation of the lower line intensity ratio found in ζ Pup. Model calculations show a noticeable amount of optical thickness for the Fe VI line, but the calculated X-ray line profile model does not show nearly enough reduction of the Fe XVII 3G line to suggest that RAD by Fe VI lines is causing the ratio anomaly in ζ Pup. Finally, I introduce preliminary steps for the analysis of XRISM spectral observations of Fe Kα lines from the starburst galaxy Messier 82. The key unsolved questions regarding M82 are what drives the hot wind and how much gas escapes the galaxy. Understanding the hot wind requires accurate measurements of its energy content, which requires obtaining constraints for the density, temperature, and velocity at the wind’s base. In order to sufficiently constrain the hot component velocity, the 6.7 keV Fe XXV line width and center must be determined to better than 10%. This accuracy requires an energy resolution ΔE ≤ 5 eV, which can be achieved by the high-resolution X-ray measurements with the XRISM Resolve calorimeter array. The M82 observation and subsequent analysis will confirm whether hot gas pressure is the primary driver of the galactic wind by measuring the energy contained in the T ∼ 10^8 K hot gas, and will constrain the mass-loading rate by measuring the velocity of the superheated nuclear gas using the Fe XXV line width. By completing these works, I will have successfully contributed to the refinement and advancement of theoretical, laboratory, and observational X-ray astrophysical data for iron transition lines.Item The Lives and Times of Stars and Black Holes in the Disks of Active Galactic Nuclei(2024) Dittmann, Alexander Joseph; Miller, Michael C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Enormous disks of gas are thought to feed the supermassive black holes at the centers of active galaxies; these disks may capture stars from nuclear clusters, or form stars in situ after collapsing under their own gravity. Such stellar populations may enrich these accretion disks with fusion byproducts, cause giant flares in these active galaxies, and leave behind compact remnants detected on earth through gravitational waves emitted as they merge with one another. This dissertation charts a theoretical expedition into these phenomena, from studying the implications of star-forming accretion disks for the growth of black holes in the early universe, to simulating the flow of gas around black hole binaries to ascertain their orbital evolution. After a brief observational and theoretical overview of stars and active galactic nuclei, this dissertation delves into the development of simplified models of accretion disk structure, including the effects of stars and black holes embedded within accretion disks. The ultimate goal of this chapter was to determine if gravitational instability in the outer regions of these accretion disks might lead to the formation of large numbers of black holes, which might go on to merge with the central supermassive black hole; this process might decrease the effective radiative efficiency of accretion onto supermassive black holes, facilitating the rapid growth of black holes in the early universe, which defies conventional explanation. Along the way, this work developed a new flavor of model to describe these disks, accounting for the pressure support provided by feedback from disk-embedded stellar-mass black holes, developed a number of semi-analytical estimates for how stars might evolve within these accretion disks, and estimated the typical timescales for objects to move through the disk. Together, these estimates showed that accelerated supermassive growth in the early universe was indeed feasible, although this estimate hinged on a number of yet-untested assumptions. Subsequently, this dissertation advances to the question of how stars evolve when embedded within hot, dense disks of gas accreting onto supermassive black holes. Moving beyond the semi-analytical models of the preceding section, the third chapter reviews simulations of stellar evolution subject to the extreme conditions within these accretion disks. Stellar evolution calculations, due to the enormous spatial and time-scales involved, are virtually always restricted to one spatial dimension. This chapter investigates a number of the ways to account for the deviations in spherical symmetry inherent to accretion disks in these calculations, before reviewing how stellar rotation and the chemical composition of these accretion disks can affect the evolution of stars embedded therein. This work developed analytical criteria governing different regimes in stellar evolution, such as the balance between the stellar accretion and nuclear burning timescales, the relationship between gas composition and gas opacity, and the limiting effect of the central supermassive black hole's gravity on stellar accretion as the two compete for gravitational influence on the gas within the disk. Ultimately, the precise, quantitative details of these simulations depend on the specific 3D-inspired prescriptions implemented, but the overall trends identified are robust. The final study presented in this dissertation investigates the feasibility of these accretion disks as the host sites for the stellar-mass black hole mergers detected by the Laser Interferometer Gravitational-Wave Observatory. One of the primary uncertainties of this scenario is whether binaries formed within the disk will tend to spiral inward after formation, or instead be driven via hydrodynamic interactions to spiral outward to the point where chaotic three-body interactions would separate the binary. To address the feasibility of this gravitational wave progenation channel, we conducted three-dimensional hydrodynamical simulations of black hole binaries embedded within these accretion disks, at orbital separations slightly smaller than the limit for dynamical instability. This chapter focused on initially circular binaries over a range of orbital inclinations with respect to the midplane of the disk, finding that binaries with orbits at all misaligned with the disk midplane are gradually realigned, and that retrograde binaries can inspiral appreciably faster than prograde ones. Although the simulations were physically incomplete, in particular neglecting magnetohydrodynamic and radiative effects, they suggest that AGN disks could indeed host the binary black hole mergers detected via gravitational waves.Item Interactions between Massive Stellar Feedback and Interstellar Gas in the Eagle Nebula(2024) Karim, Ramsey Lee; Mundy, Lee G; Pound, Marc W; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)My thesis describes multi-scale stellar feedback processes observed in the Eagle Nebula star forming region in our Milky Way galaxy. Stellar feedback from massive stars encompasses bright ultraviolet radiation which ionizes atoms and dissociates molecules in gas surrounding the stars as well as supersonic winds which impact the gas and create hot shocked layers. I study the interaction of stellar radiative and mechanical feedback with pre-existing density inhomogeneities in the molecular cloud in order to learn about the effects of the interstellar environment on the relative efficiency of various forms of feedback. This work informs our understanding of the life cycle of interstellar gas: gas forms stars and is then exposed to their winds and radiation, and we would like to know how that affects the formation of future generations of stars. The Eagle Nebula's relative proximity to us means we observe the H II region with high spatial resolution. Extra-galactic studies observe many H II regions simultaneously and at a variety of cosmic ages, but lack the resolution to understand the structure of the individual regions. High resolution studies of Galactic sources such as the Eagle serve as templates for what extra-galactic astronomers are seeing in far-away galaxies. The work also contributes to sub-grid feedback prescriptions in large-scale simulations of galaxy formation and evolution. Stars and their feedback are too small to be simulated in these contexts, so theorists require accurate approximations for the effects of stellar feedback. Massive stars form in massive molecular gas clouds and then deliver vast quantities of energy back into the clouds in the form of radiation and stellar winds. They form H II regions, 1-to-10-light-year scale areas of ionized hydrogen, which are often overpressured bubbles compared to the surrounding interstellar medium, and their supersonic winds sweep up a compressed shell of gas. Around the edge of the H II region, there lies a layer of gas which receives no >13.6 eV extreme-ultraviolet H-ionizing radiation (EUV), but is rich in 6-13.6 eV far-ultraviolet radiation (FUV) which can photodissociate molecules such as CO and H2 and ionize C. These photodissociation regions (PDRs) are heated via the photoelectric effect as FUV radiation interacts with organic molecules called polycyclic aromatic hydrocarbons (PAHs), and the regions are cooled by the collisionally excited far-infrared fine structure transitions of ionized carbon and atomic oxygen. The FEEDBACK SOFIA C+ Legacy Project (Schneider et al. 2020) studies the coupling efficiency of that energetic feedback to the gas by observing one such transition of singly ionized carbon at 158 micron referred to as C+ or [C II]. In this astrophysical context, the line is emitted primarily within PDRs. With modern heterodyne receivers and an observatory above Earth's atmosphere, we can both detect and spectroscopically resolve the [C II] line and therefore trace the morphology and kinematics of the PDR regions surrounding massive stars. We contextualize these observations with velocity-resolved observations tracing the un-illuminated molecular gas beyond the PDRs and a variety of archival data spanning the electromagnetic spectrum from radio to X-ray. I use these observations to study the Eagle Nebula, home to the iconic Pillars of Creation, and learn how pre-existing density structure evolves when exposed to stellar feedback and what that implies for the energetic coupling of the stellar feedback to the gas. My first study covers the Pillars of Creation in a detailed, multi-wavelength analysis published in the Astronomical Journal. We find that these pillars are long-lasting structures on the scale of the H II region age and that they must arise from pre-existing density structures. My second study zooms out to the greater Eagle Nebula H II region to learn how the massive stars affect the rest of the region. This analysis concludes that the primordial filamentary structure which must have led to the formation of the stellar cluster also governs the shape of the H II region and how much of the surrounding gas is affected by the feedback. Finally, I describe a software package, scoby, which I developed to aid these two studies. The software connects theoretical feedback estimates to observed star catalogs and delivers results tuned for observational studies like these. It has been used for several published analyses of other regions.Item Population Studies of Tidal Disruption Events and Their Hosts: Understanding Host Galaxy Preferences and the Origin of the Ultraviolet and Optical Emission(2024) Hammerstein, Erica; Veilleux, Sylvain; Cenko, S. Bradley; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is well-established that the majority of galaxies harbor a supermassive black hole (SMBH) in their nucleus. While some of these SMBHs are easily studied either through signatures of persistent gas-fueled accretion or direct observations of the SMBH's influence on stars and gas in its potential well, many more are elusive, providing no obvious evidence of their existence. One way to detect these dormant SMBHs is through the tidal disruption of a star that wanders too close and is torn apart under the tidal stress. These tidal disruption events (TDEs) illuminate otherwise difficult-to-study dim or distant galaxy nuclei, acting as cosmic signposts announcing the presence of the SMBH lurking there through luminous flares observed across the electromagnetic spectrum. These flares can, in principle, be used to extract information about the SMBH itself, and can therefore serve as important probes of SMBH growth and evolution. TDE host galaxies can be used to study the connection between SMBHs and their environments, an important goal in understanding the origin of SMBHs, galaxy formation, and SMBH co-evolution. My dissertation addresses both of these important facets of TDEs, their light curves and their hosts, to understand not only the events themselves but how they can be used to study SMBHs. First, I studied a sample of 30 optically selected TDEs from the Zwicky Transient Facility (ZTF), the largest sample of TDEs discovered from a single survey yet. After performing a careful light curve analysis, I uncovered several correlations between light curve parameters which indicate that the properties of the black hole are imprinted on the light curve. I also fit the light curves using tools that yield black hole mass estimates and I found no correlation between these estimates and the host galaxy stellar mass. I found no difference between the optical light curve properties, apart from the peak luminosity, of the X-ray bright and X-ray faint TDEs in this sample. This provides clues as to the origin of the optical emission and may support a scenario where the viewing angle is responsible for the observed emission. Lastly, I presented a new spectral class of TDE, TDE-featureless, which in contrast to other events, show no broad lines in their optical spectra. This new class may be connected to the rare class of jetted TDEs. Next, I studied a subset of host galaxies in the ZTF sample of TDEs. I examined their optical colors, morphology, and star-formation histories. I found that TDE hosts can be classified as ``green'', in a phase between red, inactive galaxies and blue, star-forming galaxies. Morphologically, the TDE hosts are centrally concentrated, more so than galaxies of similar mass and color. By looking at the optical spectra of the TDE hosts, which can be used to estimate the current star formation and the star formation history, I found that TDE host populations are dominated by the rare class of E+A, or post-starburst, galaxies. In tandem with the other peculiar photometric and morphological properties, this points to mergers as the likely origin for TDE hosts. I extended this study of TDE hosts by using integral field spectroscopy to infer black hole masses via the $M_{\rm BH} - \sigma_\star$ relation and investigate large-scale stellar kinematics. I found that the black hole mass distribution for TDE hosts is consistent with the theoretical prediction that they should be dominated by lower mass SBMHs. Interestingly, one TDE-featureless object was found to have a black hole mass of $\log(M_{\rm BH}/M_\odot) = 8.01$, which is likely above the Hills mass for the disruption of a solar-type star and could necessitate a rapid spin for this particular black hole. If high spin is required to launch relativistic jets, this may further support the connection between featureless TDEs and jetted TDEs. The large-scale kinematics of a galaxy are strongly tied to its merger and star formation history. I found that TDE hosts share similar kinematic properties to E+A galaxies, which are thought to be post-merger. Lastly, I presented further observations of the jetted TDE AT2022cmc. This event, discovered in the optical, presented an opportunity to place this rare class of TDE in the context of the larger TDE population. I performed a careful light curve analysis that accounts for both the thermal and non-thermal components in the light curve. I showed that the thermal component of AT2022cmc is similar to the TDE-featureless class of events and follows correlations presented for TDE light curve properties found in this thesis.Item Mechanical evolution of small solar system bodies(2023) Marohnic, Julian Charles; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a series of studies that develop and apply numerical modeling techniques to small bodies in the solar system. We are particularly interested in low-energy deformations, collisions, and disruptions, and our subjectsrange from near-Earth asteroids to Kuiper belt contact binaries in the farthest reaches of the solar system. We use the N-body code pkdgrav to investigate these processes and describe our significant additions to its capabilities. Our first subject is the Kuiper belt object Arrokoth. On January 1, 2019, the New Horizons spacecraft flew within 3,550 km of Arrokoth, returning the first in-situ images of a small body in the classical Kuiper belt. Arrokoth was found to be bilobate, with a distinctive contact binary structure. We use pkdgrav to investigate the origins of Arrokoth's striking shape and find that plausible formation mechanisms are quite limited. We rule out the possibility of a direct impact between two unbound objects and put forward an alternate scenario in which two cometesimals in a close, synchronous orbit gradually spiral in toward one another before meeting in a gentle merger. We conclude by exploring implications for the formation of small Kuiper belt objects more generally. Next, we describe our work modifying pkdgrav to accommodate non-spherical particles. Prior work in granular physics has established that particle shape is an important factor governing the behavior of granular bodies like small solar system objects. Irregular particles tend to interlock with one another, inhibiting bulk motion and adding to the shear strength of a medium. We adapt pkdgrav's existing soft-sphere, discrete element contact physics model to allow for modeling of non-spherical grains. We then apply this new capability in three, small-scale proof of concept studies of spin-up, tidal disruption, and the Brazil nut effect. We find a significant difference in behavior when comparing small rubble-pile bodies composed of spherical particles and those composed of non-spherical particles. Finally, we apply our newly-developed tools to a more comprehensive investigation of particle shape in tidal disruption simulations. We construct small rubble piles from a range of differently-shaped constituents and subject them to simulated tidal encounters with the Earth. We conduct a parameter sweep across different encounter geometries and constituent shapes and conclude that particle shape is a significant contributor to tidal encounter outcomes. The role of particle resolution is also investigated.Item Investigating the X-ray temporal and spectral properties of blazars and beamed AGN in the Swift-BAT Hard X-ray Survey(2023) Mundo, Sergio A.; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Blazars are generally known to exhibit high-amplitude, rapid variations in flux, polarization, and in their spectra across most timescales and wavelengths. While the consensus for these objects is that their emission is indeed ``highly variable", a more specific characterization of the variability may depend on the timescales considered. In this dissertation, I investigate the nature of the variability of these objects and the physical processes involved in producing it, through the lens of blazars that have been detected by the Swift Burst Alert Telescope. My foray into the high-energy astrophysics of blazars begins with a case study of a blazar-like AGN. For the first time for this source, I definitively measure X-ray reflection features and help determine the origin of its broadband X-ray emission, suggesting that the X-rays from this object predominantly come from regions in the vicinity of the black hole, while also finding evidence of jetted emission in the hard X-rays. I further explore blazar X-ray emission by investigating the rest of the blazars in the Swift-BAT survey, and in doing so I conduct the first study in the time domain dedicated to the hard X-ray variability behavior of blazars on long timescales based on ~13 years of continuous X-ray data in the 14-195 keV band. In this study, I find that a significant portion of the blazars in the sample (~37%) do not show statistically significant variability on monthly timescales, which is in tension with the expected high variability of blazars seen in previous studies. In addition, I show that for some of the brightest blazars, the long-term spectra in the hard X-rays may be described in a relatively simple way, with a power law that changes slope on monthly timescales. Since the BAT data are not sensitive to changes on shorter timescales, or to low-amplitude variability on monthly timescales, I follow up on the supposedly ``non-variable" blazars from the previous investigation by using recent NICER observations of a sub-sample of 4 such “quiescent” BAT blazars over 5 months, allowing for insight into the short-timescale and lower amplitude variability while also representing some of the longer timescales sampled by the BAT survey. I show that variations in the NICER band are in fact detected on several timescales, but that the fractional variability appears to decrease with longer timescales, implying generally low-amplitude variability across all sources and showing very low variability on monthly timescales, which is once again at odds with studies that have shown that blazars are highly variable in the X-rays on a wide range of timescales. I also show through a spectral analysis that the broadband X-ray spectra (0.3-195 keV) of these sources can be described with different power law models, with one source requiring significant absorption in the soft X-rays to fully describe its observed curvature, possibly due to absorption in the intergalactic medium. Additional observations from a new follow-up NICER campaign will further facilitate probing the variability of these BAT blazars for up to timescales of a year, serving as an additional stepping stone towards our ultimate goal of characterizing the X-ray variability of blazars and beamed AGN.Item New Messengers & New Physics: A Survey of the High-energy Universe(2023) Crnogorcevic, Milena; Ricotti, Massimo; Caputo, Regina; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Studying the origins of the high-energy emission in the Universe can profoundly affect our fundamental understanding of the cosmic origin and its evolution at the most extreme scales. In this dissertation, I explore the high-energy observations of different astrophysical systems to inform our understanding of the theoretical frameworks used to describe them. I harness the current multimessenger infrastructure to investigate questions ranging from new physics and transient astronomy to compact objects and extended emission in the gamma-ray, gravitational-wave, and neutrino skies. The focus in the first part of this dissertation is on utilizing the Fermi Large Area Telescope (LAT) low-energy (LLE) technique to search for the light axion-like-particle (ALP) within the MeV gamma-ray emission of long gamma-ray bursts (GRBs). We perform a data-driven sensitivity analysis to determine distances for which detection of an ALP signal is possible with the LLE technique, which, in contrast to the standard LAT analysis, allows for a larger effective area for energies down to 30 MeV. Assuming an ALP mass $m_a \lesssim 10^{-10}$~eV and ALP-photon coupling $g_{a\gamma} = 5.3\times 10^{-12}$ GeV$^{-1}$, we find that the distance limit ranges from $\sim\!0.5$ to $\sim\!10$~Mpc. We demonstrate that the sensitivity of the LLE technique to detecting light ALPs is comparable to the standard LAT analysis, making it an excellent complementary---yet independent---way to search for ALPs with \textit{Fermi}. Next, we select a candidate sample of twenty-four GRBs and conduct a model comparison analysis in which we consider different GRB spectral models with and without an ALP signal component. We find that including an ALP contribution does not result in any statistically significant improvement of the fits to the data. Motivated by the delay between the ALP emission time and the time of the jet break-out associated with its ordinary long-GRB emission, we conduct a novel search for ALPs within time windows that precede the main-episode gamma-ray emission of a long GRB, focusing on the sample of sources with known precursor emission detected with LAT and LLE. We report no statistically significant detection of ALPs within the GRB precursor emission and discuss the parts of the ALP parameter space probed with this method. Multimessenger astronomy is at the heart of the remainder of this dissertation. First, I present a follow-up search for excess emission of X-rays with the Swift Burst Alert Telescope (Swift-BAT) and that of gamma rays with the Fermi Gamma-ray Burst Monitor (Fermi-GBM), in spatial and temporal correspondence to gravitational-wave events reported by the LIGO/Virgo/Kagra (LVK) Collaboration. In collaboration with the Fermi-GBM Team, we combine the observations from these two instruments to determine whether there is any statistically significant excess emission around the given gravitational-wave trigger. We report no new joint detections but present the joint flux upper limits. Finally, I present the results of the cross-correlation studies between the unresolved Fermi-LAT gamma-ray and the IceCube neutrino skies. We report no positive cross-correlation in the real-data sky maps. We then combine simulation and observation techniques to place upper limits on the fraction of neutrinos produced in proton-proton or proton-gamma interactions that occur in blazars. Assuming all gamma rays from unresolved blazars are produced from neutral pions via proton-proton interactions, we find that---for energies above 10~GeV---up to 60 % of the unresolved blazar population may contribute to the diffuse neutrino background (the fraction is 30 % for proton-gamma interactions). We also include predictions for the improved sensitivity considering 20 years of IceCube data.Item Simulating Bursty and Continuous Reionization Using GPU Computing(2023) Hartley, Blake Teixeira; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Reionization is the process by which the neutral intergallactic medium of the early universe was ionized by the first galaxies, and took place somewhere between roughly redshift 30 and redshift 6, or from 100~Myr into the universe to 1~Gyr. The details of this transition are still not well understood, but observational constraints suggest that reionization happened faster than naive estimates would suggest. In this thesis, we investigate the theory that galaxies which form their stars in short bursts could complete reionization faster than galaxies which emit their photons continuously over their lifespans. We began investigating this theory with a semi-analytic model of the early universe. We used analytic methods to model the expansion of \HII (ionized hydrogen) regions around isolated galaxies, as well as the behavior of the remnant \HII regions after star formation ceases. We then compiled assortments of galaxies matching dark matter simulation profiles and associated each with an \HII region that could either grow continuously or grow quickly before entering a dormant period of recombination. These tests indicated that the remnants of bursty star formation had lower overall recombination rates than those of continuously expanding \HII regions, and that these remnants could allow for ionizing radiation from more distant sources to influence ionization earlier. We decided that the next step towards demonstrating the differences between continuous and bursty star formation would require the use of a more accurate model of the early universe. We chose a photon conserving ray tracing algorithm which follows the path of millions of rays from each galaxy and calculates the ionization rate at every point in a uniform 3D grid. The massive amount of computation required for such an algorithm led us to choose MPI as the framework for building our simulation. MPI allowed us to break the grid into 8 sub-volumes, each of which could be assigned to a node on a supercomputer. We then used CUDA to track the millions of rays, with each of the thousands of CUDA cores handling a single ray. Creating my own simulation library would afford us complete control over the distribution and time dependence of ionizing radiation emission, which is critical to isolating the effect of bursty star formation on reionization. Once we had completed, we conducted a suite of simulations across a selection of model parameters using this library. Every set of model parameters we selected corresponds to two models, one continuous and one bursty. This selection allowed us to isolate the effect of bursty star formation on the results of the simulations. We found that the effects we hoped to see were present in our simulations, and obtained simple estimates of the size of these effects.Item MULTISCALE RADIATION-MHD SIMULATIONS OF COMPACT STAR CLUSTERS(2023) He, ChongChong; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Star formation is a crucial process that lies at the center of many important topics in astrophysics: the nature of the first sources of radiation, the formation and evolution of galaxies, the synthesis of elements, and the formation of planets and life. Recent advances in computing technology have brought about unprecedented opportunities to deepen our understanding of this complex process. In this dissertation, I investigate the physics of star formation in galaxies and its role in shaping the galaxies and the Universe through numerical simulations.My exploration of star formation begins with a large set of simulations of star cluster formation from isolated turbulent Giant Molecular Clouds (GMCs) with stellar feedback using \ramses{}, a state-of-the-art radiation-magneto-hydrodynamic (radiation-MHD) code. While resolving the formation of individual stars, I have pushed the parameters (mass and density) of the simulated GMCs well beyond the limit explored in the literature. I establish physically motivated scaling relationships for the timescale and efficiency of star formation regulated by photoionization feedback. I show that this type of stellar feedback is efficient at dispersing dense molecular clouds before the onset of supernova explosions. I show that star formation in GMCs can be understood as a purely stochastic process, where instantaneous star formation follows a universal mass probability distribution, providing a definitive answer to the open question of the chronological order of low- and high-mass star formation. In a companion project, I publish the first study of the escape of ionizing photons from resolved stars in molecular clouds into the intercloud gas. I conclude that the sources of photons responsible for the epoch of reionization, one of the most important yet poorly understood stages in cosmic evolution, must have been very compact star clusters, or globular cluster progenitors, forming in dense environments different from today's galaxies. In follow-up work, I use a novel zoom-in adaptive-mesh-refinement method to simulate the formation and fragmentation of prestellar cores and resolve from GMC scales to circumstellar disk scales, achieving an unprecedented dynamic range of 18 orders of magnitude in volume in a set of radiation-MHD simulations. I show that massive stars form from the filamentary collapse of dense cores and grow to several times the core mass due to accretion from larger scales via circumstellar disks. This suggests a competitive accretion scenario of high-mass star formation, a problem that is not well understood. We find that large Keplerian disks can form in magnetically critical cores, suggesting that magnetic braking fails to prevent the formation of rotationally-supported disks, even in cores with mass-to-flux ratios close to critical. This is because the magnetic field is extremely turbulent and incoherent, reducing the effect of magnetic braking by roughly one order of magnitude compared to the perfectly aligned and coherent case, which proposes a solution to the ``magnetic braking catastrophe.''Item Photochemistry of Exoplanet Atmospheres: Modelling alien chemistry accurately and self-consistently(2023) Teal, Dillon James; Kempton, Eliza; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Exoplanets offer unique physical and chemical laboratories experiencing entirely alien environments compared to the Solar System planets. Their atmospheres, governed by the same laws of physics, display remarkable diversity and complexity. They serve as the most complex planetary phenomena we can directly observe, coupled to the planet's interior processes, formation environment, the properties of the host star, and complex chemical ecosystems. The art of modelling these systems is a rich field of study, and in this work I study the nature of photochemical models and what understanding they can provide for us based on the quality and breadth of their inputs. By characterizing the implicit uncertainty chemical models have without a well-characterized host star, I quantify the importance of host star characterization to chemical modelling, showing their sensitivity under different reaction schemes and microphysical models. I then apply this to recent observations of known exoplanet host stars LHS 3844 and AU Microscopii. Finally, I cover work to model sub-Neptune atmospheres across a wide parameter space aimed at understanding the influence of a planet's environment and unknowns on haze formation and observational prevalence in emission and transmission spectroscopy.