Evolution of supermassive black holes : the role of galaxy mergers
Abstract
In this thesis the connections between galaxy mergers and the growth of supermassive black
holes (SMBHs) are investigated. In chapter 2 we investigate the links between galaxy mergers and active galactic nuclei (AGN) as a function of luminosity, using visual classification of
galaxy images and a new measure of morphological disturbance to identify mergers, testing
the hypothesis that cataclysmic events such as mergers are required to drive the most rapid
periods of accretion. We find no correlation between merger fraction and AGN luminosity, nor
any difference in the total merger fractions of AGN and matched control galaxies. We also
explore the possibility that the role of galaxy mergers evolves through cosmic time over the
redshift range 0.5-2.2, finding a modest evolution, with higher redshift AGN more likely to be
found in mergers than controls with 99% confidence. In chapter 3 we compare samples of simulated (Illustris) galaxies to real observations, exploring the efficacy of different techniques of
merger-identification as a function of merger parameters (mass ratio, time elapsed since the
merger, stellar mass, star formation rate etc.). Detailed analysis of the ability of structural
parameters and visual classification to identify mergers allows for reinterpretation of observational studies in which AGN merger fractions are measured. We find the intrinsic merger
fractions in relevant studies are likely several times higher than reported, with 50 20% of
AGN in the local Universe triggered by galaxy mergers. In chapter 4 AGN are selected in the
optical, infrared and radio to investigate the role of mergers in triggering AGN in different
physical environments. We use structural parameters to estimate merger fractions. It is found
that infrared-selected AGN are more likely to be associated with mergers than their controls,
optically selected AGN are equivalent to controls and radio-selected AGN show fewer signs of
interactions than their controls. Collectively, this is interpreted as evidence that a substantial
fraction of SMBH growth is driven by galaxy mergers, but care needs to be taken to control
for various observational biases, which can hide or dilute real underlying causal relations.
Type
Thesis, PhD Doctor of Philosophy
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