Astronomy Theses and Dissertations

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    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.
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    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.
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    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.
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    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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    A Study of Diverse Hot Jupiter Atmospheres
    (2022) Fu, Guangwei; Kempton, Eliza; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The discovery of over 5000 planets outside the solar system has evermore changed our view of the universe and the tale of other worlds beyond Earth is no longer folklore. In 1989, David Latham found the first planetary candidate HD 114762b (Latham et al., 1989) using the radial velocity technique which was later used to discover 51 Pegasi b as the first exoplanet orbiting a solar-type star by Didier Queloz and Michel Mayor (Mayor and Queloz, 1995). Then back to December 1999, the first transiting exoplanet HD 209458b lightcurve was measured by David Charbonneau (Charbonneau et al., 2002). The gold rush of exoplanet discovery and characterization today is probably beyond the wildest dreams of the early pioneers.From the early days of discovering hot Jupiters, we are now able to study their atmospheres in detail. This thesis focuses on the study of hot Jupiter atmospheres. Hot Jupiters are rare outcomes of planet formation and their origin remains a mystery (Dawson and Johnson, 2018). The study of their atmospheres can help us to understand their formation and evolution history and also develop techniques for future remote sensing of potentially habitable transiting exoplanets in the search for life beyond earth. Just like our solar system with every planet being special and different, the same is true for hot Jupiters. I will first go into detailed studies of 4 individual hot Jupiters (WASP-76b, WASP-74b, HAT-P-41b, and KELT-20b) and then connect them to more broad population-level statistical studies. These four hot Jupiters all have dayside temperatures exceeding 2000K with an inflated radius and a very short (<10 days) orbital period. However, they exhibit different atmospheric features and properties. The relatively cooler ones like WASP-74b and HAT-P-41b show mostly blackbody-like dayside emission spectra which indicate isothermal temperature-pressure profiles. The largely featureless transmission spectrum from WASP-74b is likely caused by clouds condensing in the terminator region. On the other hand, the hotter WASP-76b dayside shows CO emission features with evidence for water thermal dissociation. Combined with heavy metal absorption features seen in the NUV part of the transmission spectrum, gaseous metals are likely causing thermal inversion in the upper atmosphere. KELT-20b shows the strongest water and CO emission features on the dayside despite a similar dayside temperature compared to WASP-76b. The unique A-type host star of KELT-20b is likely the difference-maker. The intense short-wavelength UV/NUV radiation from the A star gets preferentially absorbed in the upper atmospheres by the gaseous metals which heats up the corresponding layers and drives stronger thermal inversion. The dayside emission spectra of these four planets are then compared to all other hot Jupiters with temperatures ranging from ∼1500K to over 4000K. These four planets sit in a key transitional parameter space where we see dayside emission spectra from cooler planets mostly have water absorption features and hotter planets are mostly blackbody-like. This trend shows the cooler (<2000K) planets do not have high altitude absorbers needed for thermal inversion, and the much hotter (>3000K) ones are affected by thermal dissociation of water and rising continuum opacity of H−. Only in this in-between temperature range combined with strong inversion from gaseous metal absorbers and strong UV radiation, we could see prominent water emission features.I also did a population-level statistical study of all observed transmission spectra focused on the 1.4 μm water band. Each spectrum was fitted to determine the water feature strength which is then normalized by the planetary atmospheric scale height. I found a statistically significant trend of stronger water absorption features as a function of planet equilibrium temperature. This trend can be explained by the presence of aerosols which condense more easily under cooler conditions. Although this study was conducted back in 2017 before my other publications, the trend still holds and provides a valuable statistical comparison study framework for exoplanet atmospheres.