Astronomy Theses and Dissertations

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    (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.
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    (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.
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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.