The Enhancement of Supermassive Black Hole Accretion Rates in Post-Merger Galaxies from Cosmological Simulations
Date
2022-09-02
Authors
Byrne-Mamahit, Shoshannah
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Abstract
Numerical simulations of galaxy mergers predict that in a merger event, the angular momentum of gas is efficiently drained, leading to gaseous inflows resulting in high central gas densities and bursts of star formation. The merger-starburst connection is strongly corroborated by observational studies of interacting and post-merger galaxies. In addition to the enhancement of central star formation rate, simulations predict that gaseous inflows will increase the accretion onto central supermassive black holes (SMBHs) which may trigger or enhance nuclear activity. However, observational studies continue to debate the connection between galaxy mergers and active galactic nuclei (AGN), with contrasting conclusions on whether mergers are likely to trigger AGN and whether the majority of AGN are triggered by merger events. In this thesis, I present an analysis of the SMBH accretion rates in post-merger galaxies drawn from the IllustrisTNG, Illustris, and EAGLE cosmological simulations. The post-merger samples consist of galaxies that have experienced a merger in the last simulation snapshot in the redshift range 0<z<1, with stellar mass ratios >1:10 and post-merger stellar masses > 10^10 M_sun. I compare the post-merger SMBH accretion rates to a control sample of galaxies that are matched to the post-mergers in stellar mass, redshift, environment (and AGN feedback mode in the case of IllustrisTNG) but that have not experienced a merger (of mass ratio >1:10) within at least two Gyrs. I find that all three simulations demonstrate a population averaged enhancement in SMBH accretion rate in the post-merger sample, with accretion rates ~2 times higher than controls for IllustrisTNG and EAGLE and ~4 times higher in Illustris. In addition, I find that post-mergers are 2-4 times more likely to host a highly accreting SMBH compared with matched non-merger galaxies. In all three simulations, the population averaged SMBH accretion rate enhancement persists for ~2-3 Gyrs after coalescence, significantly exceeding the ~500 Myr lifetime of star formation rate enhancements. In the IllustrisTNG simulation, I investigate the presence of simultaneous enhancements in both the star formation and SMBH accretion rates; I find that co-incidence occurs predominantly in the first few hundred Myrs post-coalescence and depends on both the mass ratio of the merger and on the gas mass of the post-merger galaxy. Furthermore, in TNG, I find that despite the overall SMBH accretion rate enhancement, only approximately 35% of post-mergers experience a luminous AGN (L_bol>10^44 erg/s) within 500 Myrs after coalescence, and fewer than 10% achieve a luminosity in excess of L_bol>10^45 erg/s. Moreover, only ~10% of the highest luminosity (L_bol>10^45 erg/s) AGN in the IllustrisTNG galaxy sample are recent mergers. My results are therefore consistent with a picture in which mergers can (but don't always) trigger AGN activity, but where the majority of galaxies hosting high luminosity AGN are not recent mergers.