Astronomy Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2746
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Item Population Studies of Tidal Disruption Events and Their Hosts: Understanding Host Galaxy Preferences and the Origin of the Ultraviolet and Optical Emission(2024) Hammerstein, Erica; Veilleux, Sylvain; Cenko, S. Bradley; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is well-established that the majority of galaxies harbor a supermassive black hole (SMBH) in their nucleus. While some of these SMBHs are easily studied either through signatures of persistent gas-fueled accretion or direct observations of the SMBH's influence on stars and gas in its potential well, many more are elusive, providing no obvious evidence of their existence. One way to detect these dormant SMBHs is through the tidal disruption of a star that wanders too close and is torn apart under the tidal stress. These tidal disruption events (TDEs) illuminate otherwise difficult-to-study dim or distant galaxy nuclei, acting as cosmic signposts announcing the presence of the SMBH lurking there through luminous flares observed across the electromagnetic spectrum. These flares can, in principle, be used to extract information about the SMBH itself, and can therefore serve as important probes of SMBH growth and evolution. TDE host galaxies can be used to study the connection between SMBHs and their environments, an important goal in understanding the origin of SMBHs, galaxy formation, and SMBH co-evolution. My dissertation addresses both of these important facets of TDEs, their light curves and their hosts, to understand not only the events themselves but how they can be used to study SMBHs. First, I studied a sample of 30 optically selected TDEs from the Zwicky Transient Facility (ZTF), the largest sample of TDEs discovered from a single survey yet. After performing a careful light curve analysis, I uncovered several correlations between light curve parameters which indicate that the properties of the black hole are imprinted on the light curve. I also fit the light curves using tools that yield black hole mass estimates and I found no correlation between these estimates and the host galaxy stellar mass. I found no difference between the optical light curve properties, apart from the peak luminosity, of the X-ray bright and X-ray faint TDEs in this sample. This provides clues as to the origin of the optical emission and may support a scenario where the viewing angle is responsible for the observed emission. Lastly, I presented a new spectral class of TDE, TDE-featureless, which in contrast to other events, show no broad lines in their optical spectra. This new class may be connected to the rare class of jetted TDEs. Next, I studied a subset of host galaxies in the ZTF sample of TDEs. I examined their optical colors, morphology, and star-formation histories. I found that TDE hosts can be classified as ``green'', in a phase between red, inactive galaxies and blue, star-forming galaxies. Morphologically, the TDE hosts are centrally concentrated, more so than galaxies of similar mass and color. By looking at the optical spectra of the TDE hosts, which can be used to estimate the current star formation and the star formation history, I found that TDE host populations are dominated by the rare class of E+A, or post-starburst, galaxies. In tandem with the other peculiar photometric and morphological properties, this points to mergers as the likely origin for TDE hosts. I extended this study of TDE hosts by using integral field spectroscopy to infer black hole masses via the $M_{\rm BH} - \sigma_\star$ relation and investigate large-scale stellar kinematics. I found that the black hole mass distribution for TDE hosts is consistent with the theoretical prediction that they should be dominated by lower mass SBMHs. Interestingly, one TDE-featureless object was found to have a black hole mass of $\log(M_{\rm BH}/M_\odot) = 8.01$, which is likely above the Hills mass for the disruption of a solar-type star and could necessitate a rapid spin for this particular black hole. If high spin is required to launch relativistic jets, this may further support the connection between featureless TDEs and jetted TDEs. The large-scale kinematics of a galaxy are strongly tied to its merger and star formation history. I found that TDE hosts share similar kinematic properties to E+A galaxies, which are thought to be post-merger. Lastly, I presented further observations of the jetted TDE AT2022cmc. This event, discovered in the optical, presented an opportunity to place this rare class of TDE in the context of the larger TDE population. I performed a careful light curve analysis that accounts for both the thermal and non-thermal components in the light curve. I showed that the thermal component of AT2022cmc is similar to the TDE-featureless class of events and follows correlations presented for TDE light curve properties found in this thesis.Item SIFTING FOR SAPPHIRES IN THE TRANSIENT SKY: THE SEARCH FOR TIDAL DISRUPTION EVENTS IN THE OPTICAL TIME DOMAIN(2018) HUNG, TZU-YU; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tidal disruption events (TDEs) refer to the scenario where a star passes within the tidal disruption sphere of a supermassive black hole (SMBH) and becomes torn apart by tidal stresses. In the classical picture, a thermal flare is expected once the bound stellar debris circularize to form an accretion disk that feeds onto the black hole. This flare of radiation provides a unique window to study the demographics of black holes within distant and quiescent galaxies that cannot be probed by other means. In addition, TDEs serve as a powerful probe of the accretion process, where the mass fallback rate can be super-Eddington for \Mbh{} $<$ a few $\times$ 10$^7$ \Msun{}. In recent years, ground-based wide-field optical surveys have successfully detected about a dozen of TDEs. Yet our knowledge of these events is still limited due to their low occurrence rate ($\approx$ 10$^{-4}$--10$^{-5}$ gal$^{-1}$ yr$^{-1}$). In the first part of this thesis, we present results from a systematic selection of TDEs in the Intermediate Palomar Transient Factory (iPTF). Our selection targets typical optically-selected TDEs: blue transients ($g-r$ $<$ 0 mag) residing in the center of resolved red galaxies that are absent of previous nuclear activity. Our photometric selection has led to discoveries of two TDEs in $\sim$4 months, iPTF16axa and iPTF16fnl, in 2016. With the most stringent criteria, we significantly reduced the contamination rate from SN Ia and AGN from 200:1 to 4.5:1. We derived a TDE rate of 1.7$^{+2.9}_{-1.3}$ $\times$ 10$^{-4}$ gal$^{-1}$ yr$^{-1}$ and forecast a discovery rate of 32$^{+41}_{-25}$ TDEs per year for ZTF. The second part of this thesis features a detailed analysis of the photometric and spectroscopic observations on iPTF16axa. We compared iPTF16axa with 11 other TDEs in the literature with well-sampled optical light curves. We concluded that most of these TDE candidates have peak luminosities confined between log(L [erg s$^{-1}$]) = 43.4--44.4, with constant temperatures of a few $\times$ 10$^4$ K during their power-law declines, implying blackbody radii on the order of ten times the tidal disruption radius, that decrease monotonically with time. For TDE candidates with hydrogen and helium emission, the high helium-to-hydrogen ratios suggest that the emission arises from high-density gas, where nebular arguments break down. In the last part of this thesis, I present statistical analyses on the Zwicky Transient Facility (ZTF) data and comments on the TDE rate from the first few months of the survey. Finally, I close this chapter with an analysis on the optical spectra of the first ZTF TDE -- AT2018zr.