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Exploring Galactic Nuclei with Tidal Disruption Events


One of the most promising avenues for studying supermassive black holes (SMBHs) in the local Universe is through tidal disruption events (TDEs). TDEs occur when unlucky stars pass too close to a black hole and are torn apart and consumed. This often pushes the black holes to their critical Eddington limits, outshining their galactic centers and encoding the resultant light curves with a wealth of information about the disrupter and disruptee (e.g. Rees et al. 1988). Observations of TDEs are quickly becoming commonplace (e.g. van Velzen et al. 2020), with over 50 confirmed disruptions discovered in the last decade, and thousands more expected to be observed by upcoming time-domain surveys such as the Vera Rubin Observatory. These phenomena provide an exciting opportunity to study SMBHs in quiescent galaxies, the stellar populations in galactic nuclei, and the physics of black hole accretion under well defined conditions. In this thesis I develop a new model for the light curves of tidal disruption events, and combine it with data from observed tidal disruption events and dynamical models of galactic nuclei to learn about the properties and evolution of the supermassive black holes at the centers of galaxies as well as the stars that surround them.

I first describe the components of the light curve model, and use it to fit a population of observed TDEs. I show that the model can estimate the masses of the disrupting black holes as well as other properties of the system. I then use energy and efficiency estimates from the models to constrain the emission mechanism, as the source of the emission for the majority of these transients is hidden inside a layer of stellar debris and cannot be observed directly. I compare my results to similar measurements of active galactic nuclei. Next, I combine the light curve models with data from the spectra and host galaxies of TDEs to put constraints on the masses of disrupted stars for several events. I find that they have high nitrogen-to-carbon abundances, implying stellar masses $\gtrsim 1 - 2 M_\odot$. These `moderately massive' stars are over-represented by a factor of $\gtrsim 10^2$ compared to the overall stellar population of the hosts. Finally, I explore how SMBH binaries can increase the rates of tidal disruptions in post-merger galaxies, and how TDEs can, in turn, help us discover SMBH binaries.

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