New Methods for Tracking Galaxy and Black Hole Evolution Using Post-Starburst Galaxies

Abstract

Galaxies in transition from star-forming to quiescence are a natural laboratory for exploring the processes responsible for this evolution. Using a sample of post-starburst galaxies identified to have recently experienced a recent burst of star formation that has now ended, I explore both the fate of the molecular gas that drives star formation and the increased rate of stars disrupted by the central supermassive black hole. Chapter 1 provides an introduction to galaxy evolution through the post-starburst phase and to tidal disruption events, which surprisingly favor post-starburst galaxy hosts. In Chapter 2, I present a survey of the molecular gas properties of 32 post-starburst galaxies traced by CO (1--0) and CO (2--1). In order to accurately put galaxies on an evolutionary sequence, we must select likely progenitors and descendants. We do this by identifying galaxies with similar starburst properties, such as the amount of mass produced in the burst and the burst duration. In Chapter 3, I describe a method to determine the starburst properties and the time elapsed since the starburst ended, and discuss trends in the molecular gas properties of these galaxies with time. In Chapter 4, I present the results of followup observations with ALMA of HCN (1--0) and HCO+ (1--0) in two post-starburst galaxies. CO (1--0) is detected in over half (17/32) the post-starburst sample and the molecular gas mass traced by CO declines on ~100 Myr timescales after the starburst has ended. HCN (1--0) is not detected in either galaxy targeted, indicating the post-starbursts are now quiescent because of a lack of the denser molecular gas traced by HCN. In Chapter 5 I quantify the increase in TDE rate in quiescent galaxies with strong Balmer absorption to be 30-200x higher than in normal galaxies. Using the stellar population fitting method from Chapter 3, I examine possible reasons for the increased TDE rate in post-starburst galaxies in Chapter 6. The TDE rate could be boosted due to a binary supermassive black hole coalescing after a major merger or an increased density of stars or gas remaining near the nucleus after the starburst has ended. In Chapter 7, I present a summary of the findings of this dissertation and an outlook for future work.