Astronomy Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2746

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CONNECTING THEORY AND OBSERVATIONS OF EXOPLANET ATMOSPHERES AND SURFACES AT THE INDIVIDUAL AND POPULATION LEVEL WITH JWST
(2024) Ih, Jegug; Kempton, Eliza M.-R.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Observing an exoplanet’s atmosphere via photometry and spectroscopy has provided the main window to understanding its properties and processes, as the atmospheric spectra encompass information about the chemistry, thermal structure, surfaces, as well as formation history and even biology. To this end, one key science goal of the James Webb Space Telescope (JWST) is to establish whether rocky planets around M dwarfs can host atmospheres or not. JWST offers unprecedented signal-to-noise and unlocks new parameter space regimes of planets available for characterizing not only the atmosphere but also the surface. This advancement in observing capability simultaneously poses novel challenges to atmospheric characterization. My dissertation addresses some of the new challenges to atmospheric retrievals in the era of JWST and the characterization of rocky planets. Firstly, I quantified the effects of wavelength-correlated systematics on atmospheric retrievals. Wavelength-correlated noise can occur due to instrumental systematics or stellar effects and the merging of discrete data sets. I investigated the effect of correlated noise and constrained the potential biases incurred in the retrieved posteriors by performing retrievals on multiple noise instances of synthetic data. The study found that correlated noise allows for overfitting the spectrum, thereby yielding a better goodness of fit on average but degrading the overall accuracy of retrievals by roughly 1σ. In particular, correlated noise can manifest as an apparent non-Rayleigh slope in the optical range, leading to an incorrect estimate of cloud/haze parameters. Finally, I show that while correlated noise cannot be reliably distinguished with Hubble Space Telescope observations, inferring its presence and strength may be possible with JWST. Secondly, I studied the how the choice in parameterization of the atmospheric composition can influence the posterior when performing retrieval analyses on terrestrial planet atmospheres, for which the mean molecular weight is not known a priori. By performing self-retrievals and varying the parameterization, I found that the centered log-ratio transform, commonly used for this application, tends to overestimate the abundances of spectroscopically active gases when inactive ones are present. Over multiple noise instances, I found that no one parameterization method always outperforms others. Comparing the Bayesian evidences from retrievals on multiple noise instances, I found that for a given spectrum, the choice in parameterization can affect the Bayes factor of whether a molecule should be included. Alongside astrophysical effects, this remains a fundamental challenge to atmospheric retrievals for small planet and can addressed by more observations. Finally, I constrained the atmospheric thickness and characterized the surface from the first JWST measurement of thermal emission from a rocky exoplanet, TRAPPIST-1 b. I compared TRAPPIST-1 b’s measured secondary eclipse depth to predictions from a suite of self-consistent radiative-convective equilibrium models. I found that plausible atmospheres (i.e., those that contain at least 100 ppm CO2) with surface pressures greater than 0.3 bar are ruled out at 3σ, regardless of the choice of background atmosphere, and a Mars-like thin atmosphere with surface pressure 6.5 mbar composed entirely of CO2 is also ruled out at 3σ. I modelled the emission spectra for bare-rock planets of various compositions and found that a basaltic surface best matches the measured eclipse depth to within 1σ.
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.
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.
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.
THE FORMATION OF METAL-FREE POPULATION III STARS IN X-RAY AND LYMAN-WERNER RADIATION BACKGROUNDS
(2024) Park, Jongwon; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Metal-free Population III (or Pop III) stars are instrumental in shaping the early universe, influencing the formation of the first galaxies. The formation of Pop III stars depends on the fraction of molecular hydrogen (H2), which is regulated by X-ray and Lyman-Werner (LW) radiation backgrounds. Therefore, gaining insight into the impact of these radiation backgrounds is essential for unraveling the mysteries surrounding Pop III star formation and their impacts on the first galaxies. In this dissertation, I investigate the interaction between X-ray/LW backgrounds and the formation of Pop III stars. To conduct this investigation, I employed the radiative hydrodynamics code RAMSES - RT. I implemented various physical processes governing Pop III star formation, such as primordial chemistry, radiation background, secondary ionization/heating, and self-shielding. Performing a grid of simulations covering a large parameter space of X-ray/LW intensity, I systematically explored the effects of radiation backgrounds on Pop III stars. I found that a moderate X-ray background boosts the H2 fraction in dark matter halos, facilitating Pop III star formation in low-mass halos. In contrast, a LW background dissociates H2 and prevents star formation in low-mass halos. This result suggests that the number of Pop III supernovae detected by the JWST is enhanced by an X-ray background. Furthermore, I discovered that an X-ray background reduces the characteristic mass and multiplicity of Pop III stars. This leads to a top-heavier initial mass function and may have a potential impact on galaxy formation. Moreover, I made further improvements to the simulations by incorporating radiative feedback from Pop III protostars. This study confirmed previous works that radiation from protostars suppresses their growth, thereby playing a significant role in determining the mass of Pop III stars theoretically. I also found that hierarchical binaries (binaries of binaries), eccentric orbits, and outward migration are common occurrences in Pop III star formation. Eccentric orbits induce variability of Pop III protostars and this is observable by the JWST when light is magnified through gravitational lensing. In a follow-up study, I investigated the origin of outward migration and found that the gas disks around the protostars accrete gas with high angular momentum and transfer the angular momentum to the binary stars through torques. This finding paves the wayfor studies of migration behaviors across different stellar populations. Finally, I explored the X-ray effects on the number of Pop III stars using cosmological simulations. Developing methods to calculate the intensity of the radiation background on the fly and realistically accounting for the X-ray feedback loop, I found that a weak X-ray background develops and this background ionizes the intergalactic medium, thereby moderately increasing the number density of Pop III stars (by a factor of ∼ 2). This rise in the number of Pop III stars due to X-ray radiation lowers the star formation rate of metal-enriched Pop II stars, highlighting the significance of the X-ray background in galaxy formation.This thesis covers various aspects of Pop III star formation and the effect of X-ray radiation backgrounds which has been overlooked by previous studies. It lays a foundation for future research aimed at connecting the theoretical understanding of Pop III star formation and observations targeting Pop III stars and the first galaxies.
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.
The Shadows of Would-Be Gods: Finding Transiting Jovians, Terrestrials, and Everything in Between with TESS to Understand Hot Jupiter Formation and the Best Targets for JWST
(2023) Hord, Benjamin James; Kempton, Eliza; Colón, Knicole; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
NASA’s Transiting Exoplanet Survey Satellite (TESS) mission launched in 2018 and has since observed more than 90% of the sky and discovered more than 6,000 planet candidates of many sizes, temperatures, and orbital periods. Hot Jupiters, in particular, have benefited from TESS since these planets are uniformly distributed throughout the sky and produce large transit signals. Many questions remain about this enigmatic class of large gas giants orbiting extremely close to their host stars regarding their formation and evolution. My dissertation leverages TESS to investigate the potential formation mechanisms of hot Jupiters and applies relevant planet discovery techniques to a collection of planet candidates that would be most amenable, or “best-in-class,” for atmospheric characterization with JWST. First, I performed a uniform search for nearby companion planets to hot Jupiters observed by TESS in its first year of operations. The lack of planets nearby hot Jupiters in their planetary systems has long been thought to be a fingerprint of their dynamically active formation history, although a recent set of discoveries of nearby planets in three hot Jupiter systems has challenged this notion. I developed a custom-built search, vetting, and validation pipeline to detect additional transit signals in TESS light curves of hot Jupiter systems and evaluate the planetary nature of each. This study found a host of new transit-like signals but none were deemed to be caused by planets, reinforcing the idea that companion planets to hot Jupiters are rare. I also estimated the expected rate at which hot Jupiters should have companions and found it to be 7.3+15.2−7.3%. Second, I continued the search for additional planets in hot Jupiter systems as TESS continued to observe the sky and discovered a new signal in the WASP-132 system. I vetted and statistically validated this signal to demonstrate that it is indeed from a new planet, dubbed WASP-132c. This planet orbits interior to the hot Jupiter WASP-132 b and constitutes only the fourth such system discovered at the time. I performed some initial analysis on the limited sample of hot Jupiters with nearby companions and found evidence suggesting that systems with this architecture predominantly have an outer hot Jupiter beyond the ∼3 day orbital period pileup with an inner companion. This may be due to a number of factors, including physical and observational, such as formation mechanism or the bias towards short period planets of transit surveys. Finally, I leveraged the planet discovery, vetting, and validation techniques I had applied to the search for companions to hot Jupiters to perform a large-scale validation of over 100 planet candidates discovered by TESS that were deemed “best-in-class” for atmospheric characterization with JWST. This included the synthesis and ranking of all planets and planet candidates by observability with JWST into a single sample and then performing vetting and validation analyses on those that were candidates. In total, I statistically validated 22 planet candidates and marginally validated a further 35. I present the final best-in-class sample as a community resource for future JWST observations.
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.
A Song of Fire & Ice: Evolutionary Properties of Hot & Cold Small Bodies
(2023) Holt, Carrie; Knight, Matthew M.; Richardson, Derek C.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Small bodies (i.e., asteroids and comets) play an important role in our understanding of the Solar System. They are composed of the same planetesimal material that was incorporated into the planets, but their smaller size kept them from experiencing extensive processing (such as differentiation or atmosphere-related surface erosion). Therefore, their primitive nature allows us to probe the composition of the early Solar System and its subsequent evolution. Even though comets and asteroids are expected to contain material characteristic of their formation region, they have undoubtedly undergone some degree of processing since they were formed. The overarching motivation for small-body science is to disentangle primordial characteristics from evolutionary characteristics developed since formation with the goal of better understanding how our Solar System came to be. This work seeks to tackle a small piece of this goal by studying the objects of two extreme populations: the most and least thermally processed bodies. This thesis uses ground-based broadband optical photometry to investigate the differences between different small body populations and how thermal processing alters the characteristics of objects over time. First, we investigate the optical colors of near-Sun asteroids that experience extreme temperatures of > 1000 K to better understand the dominant processes that affect their surface properties and could potentially lead to their disruption. Next, we characterize the long-term brightness evolution of long-period comets using two distinct datasets: 1) an observing campaign that conducts long-term monitoring of long-period comets that are active beyond the region where water-ice sublimation is efficient, and 2) photometric magnitudes of long-period comets with well-characterized orbits that were collected and reported by amateur observers. We assess our ability to improve brightness predictions for comets discovered at large heliocentric distances and establish if brightness behavior can be used as a diagnostic of dynamical age.
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.
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.
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.''
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.
ANALYZING THE STAR FORMATION EFFICIENCY AND PHYSICAL CONDITIONS OF THE MOLECULAR GAS IN NEARBY GALAXIES
(2023) Villanueva, Vicente; Bolatto, Alberto D; Vogel, Stuart N; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Star formation activity plays a key role in driving galaxy evolution, and it depends on the amount of cold gas available (as traced by CO emission) and on the physical conditions and dynamical state of this gas. This work focuses on investigating the star formation efficiency of the gas, both molecular and total, as a function of local and global galaxy properties. The galaxy samples studied in this thesis are representative of the nearby universe, and we use a combination of interferometric CO observations and integral field unit optical spectroscopy for most of our analyses. First, we show that in a sample of galaxies dominated by ``field galaxies'' the disk scale lengths for the molecular and stellar components are very similar, reflecting the close relation between CO emission and star formation activity. Our analysis of the radial dependence of the star formation efficiency of the total gas on morphological, structural, and dynamical properties of the galaxies shows that there is a smooth, continuous exponential decline with increasing radius (mostly driven by the increased contribution of atomic gas), and a systematic increase in the average gas efficiency from early to late type galaxies. Our results also show a morphological dependence of the efficiency per orbital time, which may reflect star formation quenching due to the presence of a bulge. Next, we test the impact of environmental processes on galaxies immersed in the rich environment of the Virgo cluster. We show that in these galaxies the CO emission is more centrally concentrated than the stars, unlike what we saw in the field. Moreover, in the central regions of galaxies with an increasing level of perturbation (as determined by the morphology and kinematics of their atomic gas emission), the mean molecular-to-atomic gas ratio increases while the star formation efficiency of the molecular gas in the same region decreases. This demonstrates that the cluster environment not only affects the outskirts of galaxy disks and their atomic gas, but deeply changes the distribution and efficiency of the centrally located molecular gas component. Finally, we study the onset of star formation cessation in galaxies (``quenching'') by investigating a complete sample of galaxies spanning from the main sequence (normal star forming objects) to the green valley (galaxies which are starting to quench) to the red cloud (galaxies that are mostly quiescent, that is, ``red and dead'' objects). We find that the star formation activity and the molecular gas-to-stellar mass ratio track each other. We also note that green valley galaxies have lower molecular star formation efficiencies than galaxies on the main sequence. On average, we find that within the bulges of green valley galaxies the molecular gas star formation efficiency is lower than in main sequence galaxies. Also in green valley galaxies, we find that the molecular gas to stellar ratio, the molecular gas star formation efficiency, and the specific star formation rate all increase with increasing distance to the center. Our results suggest that gas depletion or removal does not fully explain the star-formation quenching in galaxies transiting through the green valley, and that a reduction in star formation efficiency is also required during this stage. This is reminiscent of the so-called ``morphological quenching.''
MOLECULAR SPECTROSCOPY OF STAR FORMING REGIONS: COOL AND HOT, CLOSE AND FAR
(2023) Li, Jialu; Harris, Andrew; Tielens, Alexander; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Star formation processes originating from dense molecular clouds leave us a molecular universe. How molecules probe the physical conditions at different star-forming stages and how the physical environments control the formation of the chemical inventory becomes a key question to pursue. In the past, the understanding of this problem is impeded by instrumentallimitations. With instruments advanced in sensitivity and spatial/spectral resolution, this thesis investigates the molecular environment of different star-forming regions. Half of this thesis (Chapter 2 and Appendix A) focuses on mapping cold dense molecular gas in an external galaxy, IC 342, at 3 Mpc. The distribution of molecular gas was efficiently mapped with a set of density-sensitive tracers with Argus. Argus is the first array receiver functioning at 3 mm on the 100 m Green Bank Telescope (GBT) and provides a resolution of 6′′–10′′. As this study was conducted in the early era of Argus’ deployment, valuable information on the instrument’s behavior is learned. The resolved molecular maps characterize the fundamental physical properties of the clouds including the volume density and the excitation conditions. Comparisons with results from radiative transfer modeling with RADEX help to decrypt this information. The high spatial resolution of Argus also provides an opportunity in inspecting a scale-scatter breakdown of the gas density-star formation correlation in nearby galaxies and in investigating the influence of a finer spatial resolution on the correlation. The other half of the thesis (Chapters 3 and 4) studies the hot core, an embedded phase during massive star formation, of a proto-binary system W3 IRS 5 at 2.2 kpc. Rovibrational transitions of gaseous H2O, CO, and isotopologues of CO were detected with mid-IR absorption spectroscopy. The high spectral resolution (R ∼50,000–80,000) not only separates each transition individually but also decomposes different kinematic components residing in the system with a velocity resolution of a few km/s . Physical substructures such as the foreground cloud, high-speed “bullet”, and hot clumps in the disk surface are identified. Characterization of the physical substructures is conducted via the rotation diagram analysis and curve-of-growth analyses. The curve-of-growth analyses, under either a foreground slab model or a disk model, take account of the optical depth effects and correct the derived column densities by up to two orders of magnitude. The disk model specifically suggests a disk scenario with vertically-decreasing temperature from mid-plane, which is intrinsically different from externally illuminated disks in the low-mass protostellar systems that have hot surfaces. Connections between physicalsubstructures and chemical substructures were also established. Investigations on chemical abundances along the line of sight reveal the elemental carbon and oxygen depletion problem.
PHYSICAL CONDITIONS OF THE MULTI-PHASE INTERSTELLAR MEDIUM IN NEARBY GALAXIES FROM INFRARED AND MILLIMETER-WAVE SPECTROSCOPY
(2022) Tarantino, Elizabeth; Bolatto, Alberto D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Gas and dust in the interstellar medium (ISM) cools and condenses, gravitationally collapses, and forms stars. At the same time, stars can heat and ionize their surroundings, influencing the physical conditions of the nearby ISM. In this thesis, I take a multi-wavelength, spectroscopic approach to investigate the physical conditions of the multi-phase ISM in nearby galaxies. The [CII] fine-structure transition at 158 micron is frequently the brightest far-infrared line in galaxies and can trace the ionized, atomic, and molecular phases of the ISM. I present velocity-resolved [CII] observations from SOFIA in the nearby galaxies M101 and NGC 6946 and determine that [CII] emission is associated with the atomic and molecular gas about equally, with little contribution from the ionized gas. Using the [CII] cooling function, I calculate the thermal pressure of the cold neutral medium and find that the high star formation rates in our sample can drive large thermal pressures, consistent with predictions from analytical theory. Next, I investigate the properties of the ionized gas around one of the hottest and most luminous Wolf-Rayet (WR) stars in the Small Magellanic Cloud. I use spatially resolved mid-infrared Spitzer and far-infrared Herschel spectroscopy to establish the physical conditions of the ionized gas. Using the photoionization code Cloudy, I construct models with a range of constant densities between n_H = 4 - 12 cm^-3 and a stellar wind-blown cavity of 15 pc that reproduce the intensity and spatial distribution of most ionized gas emission lines. The higher ionization lines cannot be produced by the models --- however, I show that wind-driven shocks or a harder ionizing WR spectrum can explain their intensities. Lastly, I explore the properties of molecular clouds in a large (170x350 pc) map of an active star-forming region in the Large Magellanic Cloud. Using 12CO(2-1) and 13CO(2-1) observations from the ALMA ACA, I decompose the emission into individual cloud structures and determine their sizes, linewidths, mass surface densities, and virial parameters. Almost all of the clouds are gravitationally bound or marginally bound and share similar properties to molecular clouds in the Milky Way. I do not find evidence that the surrounding star formation significantly influences the kinematic properties of the clouds through stellar feedback.
On the Dynamics of Binary Asteroids Applied to DART Mission Target (65803) Didymos
(2022) Agrusa, Harrison Fitzgerald; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
NASA’s Double Asteroid Redirection Test (DART) mission will be the first full-scale demonstration of a kinetic impactor for planetary defense. On September 26, 2022, the DART spacecraft is expected to impact Dimorphos, the secondary component of the Didymos binary asteroid system. The DART impact will reduce Dimorphos’s relative orbital velocity, shrinking both its semimajor axis and orbit period. The mutual orbit period will then be measured us- ing ground- and space-based observations in order to deduce the momentum transfer efficiency, which is an important parameter in planetary defense that has never been measured experimentally at a realistic scale. This thesis comprises a set of studies on the spin and orbital dynamics of the Didymos system conducted in support of the DART mission. Owing to the close proximity of Didymos and Dimorphos and their irregular shapes, the mutual dynamics are non-Keplerian and exhibit a high degree of spin-orbit coupling, which often requires the use of specialized numerical methods to model the system. First, we conducted a benchmarking and sensitivity study to identify the best simulation codes for future DART-supported studies and to understand how small perturbations in the initial conditions can affect the resulting dynamical evolution of the system. Then, we demonstrated that Dimorphos can enter a wide range of post-impact spin states, including possible chaotic non-principal axis rotation, depending on its shape and the amount of momentum transferred by the DART impact. We then explored the implications of an excited spin state, including the possibility of ongoing granular motion on Dimorphos’s surface resulting from the orbital perturbation induced by the DART impact. This thesis is focused predominantly on the dynamics of the Didymos binary. However, there are many other binary systems among the near-Earth asteroid population with similar physical and dynamical properties, making the results presented here relevant to the NEA binary population in general.
A portrait of the binary compact merger as a young: Short GRB, Gravitational wave, Afterglow, and Kilonova
(2022) Ahumada Mena, Tomas Fernando; Singer, Leo P; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Both binary neutron star (BNS) and neutron star–black hole (NSBH) mergers radiate gravitational waves (GWs) as they gradually spiral inwards. Once they merge, they emit electromagnetic (EM) radiation that is potentially detectable across the entire EM spectrum, from hours to years after the coalescence. Right after the merger, a short burst of gamma-rays is followed by an hours to days long optical/near infrared (NIR) transient (i.e. kilonova (KN)), which is powered by the decay of the r-process nucleosynthesis elements. Depending on the angle of the gamma-ray burst (GRB) relative to Earth, a seconds to years long afterglow can be detected from radio to X-rays. The EM radiation from these mergers has shed light into different fields of physics and astronomy: they are sources of GWs, a site of rapid neutron-capture process (r-process) nucleosynthesis, and promising standard candles. However only one BNS merger has been studied in detail: AT2017gfo, the EM counterpart to GRB 170817A/GW170817. This thesis focuses on the opticalsearches of these multi-messenger sources using wide field of view (FOV) telescopes. The first chapter of this thesis describes the systematic search for optical counterparts to short gamma-ray bursts (SGRBs). We used the Zwicky Transient Facility (ZTF) to follow-up 10 short duration GRBs detected by the Fermi Gamma- ray Burst Monitor (GBM). We covered areas between 250 and 3000 deg2, and followed-up more than 60 objects, photometrically and spectroscopically. While we did not find a counterpart to a compact binary merger, we used the ZTF magnitude limits (i.e. ∼ 21 mag in the r-band) to compare to SGRB afterglows and KN models, to show that our searches are sensitive to most KN models up to 200 Mpc. However, the majority of SGRB afterglows in the literature have been found at relatively higher redshifts (i.e. mean z ∼ 0.5), making them fainter than our magnitude limits. Moreover, we explore the efficiency of our searches and we determine our searches have probed between redshift 0.16-0.4, depending on the energy models assumed for the SGRBs. Future campaigns can expand the horizon to redshift 0.2-0.7. The second part of this thesis is about the discovery of the shortest gamma-ray burst coming from a collapsing massive star. In the context of the optical follow-up of short GRBs with ZTF, we triggered target-of-opportunity (ToO) observations in the error region of GRB 200826A, a 1.13 sec duration GRB. There we found the afterglow of the burst, ZTF20abwysqy, with an optical decay rate ∼ 1 mag/day. The afterglow was additionally X-ray and radio bright. At the redshift of the host galaxy z = 0.74 , its hardness - intensity relation (i.e. Epeak,z − Eγ,iso) is consistent with the long GRB population, puzzling the community. We present the afterglow and host galaxy analysis, along with Gemini Multi-Object Spectrograph (GMOS) observations that show a rising source in the i-band that could only be explained by an underlying supernova. The third chapter of the thesis describes the optical follow-up of gravitational wave events using the ZTF. We describe the observing strategy, as well as the selection and monitoring of GW counterpart candidates. Our ToO strategy allowed us to sift through ∼ 2 million sources to select ∼ 160 candidates for follow-up. We apply this strategy to search for 13 GW alerts during the third LIGO/Virgo observing run (O3). Particularly, we describe the case of the first BNS merger in O3, S190425z, and two NSBH mergers, S200105ae and S200115j. As no counterpart was found for any of the GW events, we use the photometric limits of our searches to compare to KN models. Finally, we explore how the upcoming Rubin Observatory will be able to serendipitously find KNe, independently from GW or SGRB triggers. For this, we simulated the universe accessible to the survey and use it to derive contamination rates for different classes of transients. When using a filtering scheme based on the magnitude evolution of the sources, we find that ∼90% of the sources that fade faster than 0.4 mag/day are either GRB afterglows or supernova (SN) IIb shock breakout. This strategy is only capable of retrieving ∼3% of the generated KNe, mainly due to the fast fading nature of the KNe and their intrinsic low luminosity. We propose that future filtering schemes should take into consideration not just the detections, but the difference in magnitudes, ∆m, between the last detection and the subsequent limiting magnitude. Additional information as color, host galaxy or NIR counterparts on future NIR surveys could also improve the selection.
Tracing the formation and merger-driven growth of massive black holes with the Zwicky Transient Facility
(2022) Ward, Charlotte Alison; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The dawning of low--frequency gravitational wave (GW) astronomy via pulsar timing arrays and space--based GW interferometry will provide new opportunities for the study of the supermassive black hole (SMBH) binaries which form as galaxies merge through cosmic time. The onset of observational GW studies has coincided with the expansion of wide--field optical time--domain surveys such as the Zwicky Transient Facility (ZTF), which provide a complementary way to detect and analyze SMBHs when they accrete gas and emit at optical wavelengths. In this thesis I describe how high cadence surveys like ZTF can be used to discover rare populations of massive black holes which inform our understanding of early massive black hole seeding channels and their subsequent growth through mergers to produce the SMBH populations we see today. In the first part of this thesis I present a search for variable active galactic nuclei (AGN) which are spatially offset from their host galaxies using time-resolved imaging data from ZTF and deeper, higher resolution imaging data from the Legacy Surveys. I present a population of 52 variable AGN in merging galaxies in addition to 9 candidates for gravitational wave recoil of remnant SMBHs which may be used to constrain SMBH binary merger rates and spin alignment efficiencies. I also examine the dramatic rebrightening of a previous recoiling SMBH candidate SDSS1133, and conclude from spectroscopic follow--up that it is more likely an outbursting luminous blue variable star. In the second part of the thesis, I present a population of 190 low--mass AGN in dwarf galaxies discovered by their optical or mid--infrared variability in deep ZTF difference imaging and forward--modeled photometry of {\it WISE} image stacks. These intermediate mass black hole (IMBH) candidates can be used to constrain the low--mass end of the $M_{BH}-\sigma_*$ relation and dwarf galaxy occupation fractions in order to better understand the origins of the first massive black holes. Only $9$ candidates from my search had been detected previously in radio, X-ray, and variability searches for dwarf galaxy AGN. I find that spectroscopic approaches to AGN identification would have missed 81\% of my ZTF IMBH candidates and 69\% of my {\it WISE} IMBH candidates, showing the promise of variability searches for discovery of otherwise hidden low--mass AGN. In the third part of this work, I present 299 variable AGN in ZTF which have double--peaked Balmer broad lines from the motion of gas in their accretion disk, increasing the number of known double--peaked emitters (DPEs) by a factor of $\sim$2. DPEs can arise as false positive candidates in both spectroscopic and variability--based searches for SMBH binaries, so it is important to characterize the properties of their spectra and light curves. I find that 16\% of variable broad line AGN in ZTF are DPEs and that $\sim$50\% of the DPEs display dramatic changes in the relative fluxes of their red and blue peaks over long $10-20$ year timescales. I show that a number of DPEs exhibit apparently periodic and chirping signals in the optical and mid--infrared and discuss how this arises naturally from their power spectra. I show that DPE light curves have slightly steeper power spectra than their standard broad line counterparts and are $\sim$1.5 times more likely to have a low frequency turnover. I compare the variability and spectroscopic properties of the ZTF DPE population with the recently discovered inspiraling SMBH binary candidate SDSSJ1430+2303 (ZTF18aarippg) and conclude that the variable velocity--offset broad lines and periodic behavior of ZTF18aarippg are not unusual compared to other DPEs, and it is therefore more likely to be a single AGN rather than an SMBH binary. I conclude this thesis by outlining how the transient detection and image forward--modeling techniques presented in this thesis can be used to find populations of low accretion rate, off--nuclear AGN with the upcoming Legacy Survey of Space and Time at the Vera Rubin observatory in order to produce much better constraints on massive black hole seeding channels and GW recoil rates. I also discuss how these techniques can be applied to new science cases, such as the analysis of strongly gravitationally lensed supernovae and quasars, for cosmological studies with LSST.
ACTIVE GALACTIC NUCLEUS FEEDBACK IN GIANTS AND DWARFS
(2022) Liu, Weizhe; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Feedback from Active Galactic Nucleus (AGN) may play a critical role in the coevolution of galaxies and supermassive black holes (SMBH). Specifically, AGN feedback may quench star formation, suppress baryon-to-dark matter mass ratio, shape galaxy morphology, impact the circumgalactic (CGM)/intergalactic medium (IGM), and regulate SMBH accretion. One of the most important forms of AGN feedback is powerful, quasar/AGN-driven outflows. However, the physical details of these outflows, including their driving mechanism and spatial extent, are still not well constrained. In addition, while these outflows are believed to be effective in massive galaxies, their impact on dwarf galaxies (M⋆ ~10^9.5 M⊙) remains largely unknown. To answer these open questions, my thesis focuses on AGN feedback via quasar/AGN-driven outflows in both ultraluminous infrared galaxies (ULIRGs) and dwarf galaxies with four projects.In the first half of my thesis, I examine the outflows in nearby ULIRGs with two objectives: In Chapter 2, I present a dedicated investigation of the highly ionized, likely pc-scale quasar/AGN winds in a sample of 21 nearby ULIRGs through HST/COS far-ultraviolet (FUV) spectroscopy. Blueshifted Lyα emission is prevalent in the sample, which is probably closely related to the outflowing gas and AGN activity in these objects. Additionally, the Lyα escape fractions tend to be slightly larger in objects with stronger AGN and larger outflow velocities. Highly ionized O VI and N V outflows are detected in a coherently selected, AGN-dominated ULIRG sample for the first time. Together with the results from a matched quasar sample, these outflows show higher incidence rates and larger EW and velocities in X-ray weak sources and sources with high X-ray absorbing column densities, implying that these outflows are radiatively-driven; In Chapter 3, I describe a deep, Chandra imaging spectroscopy study of the nearby ULIRG Mrk 273. The data have revealed a ∼40 kpc×40 kpc X-ray nebula, which is relatively hot and has a super-solar α/Fe abundance ratio. This nebula is most likely heated and metal-enriched by outflows over time. Additionally, the existence of a dual AGN is strongly suggested by the data, and extended 1–3 keV emission are detected, likely related to the AGN photoionized gas and/or outflowing gas. In the second half of my thesis, I turn to look at the AGN-driven outflows in dwarf galaxies: In Chapter 4, I report the results from a dedicated optical integral field spectroscopic study of a sample of eight dwarf galaxies with known AGN and suspected outflows. Fast, kpc-scale outflows are detected in seven of them. The outflows show 50-percentile (median) velocity of up to ∼240 km s^−1 and 80-percentile line width reaching ∼1200 km s^−1, in clear contrast with the more quiescent kinematics of the host gas and stellar components. The kinematics and energetics of these outflows suggest that they are primarily driven by the AGN. A small but non-negligible portion of the outflowing material likely escapes the main body of the host galaxy and contributes to the enrichment of the circumgalactic medium. The impact of these outflows on their dwarf host galaxies is similar to those taking place in the more luminous AGN with massive hosts in the low-redshift universe. In Chapter 5, I discuss the results from a pilot HST/COS spectroscopy program to examine three objects studied in Chapter 4. Blueshifted absorption features tracing fast outflows are detected in two of the three objects. For object J0954+47, the outflow is detected in multiple ions and is much faster than those in star-forming galaxies with similar star formation rates. The outflow velocity exceeds the escape velocity of this system, suggesting that a large fraction of the outflowing gas may escape. The outflow carries significant amount of mass, momentum and kinetic energy, which may transport material out of the galaxy more efficiently than the gas consumption by star formation. The ratio of kinetic energy outflow rate to AGN luminosity of this outflow is at least comparable to the expectation from simulations of AGN feedback.Finally, in Chapter 6, I summarize the main results of the whole thesis, and briefly highlight several future works that may lead to a more comprehensive understanding of AGN feedback in ULIRGs and dwarf galaxies.

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