Minor gravitational interactions as contributors to supermassive black hole growth

Craig, Jess and Simmons, Brooke (2020) Minor gravitational interactions as contributors to supermassive black hole growth. Masters thesis, Lancaster University.

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Abstract

Galaxies have existed for the majority of the lifetime of the Universe, having first formed 400-1000 Myr after the Big Bang, and it is known that most galaxies contain supermassive black holes (SMBHs) at their centres. In recent years, correlations have been found between the evolution of galaxies and that of their central SMBHs. Therefore, understanding the evolution of SMBHs is key to our understanding of the evolution of their host galaxies and that of the Universe. Early studies have shown a link between merger activity in galaxies and the growth of their SMBHs; however, more recent studies have shown that merger-free processes and very minor interactions provide an important additional pathway to SMBH growth. We investigate a relatively unexplored possible pathway to SMBH growth: minor gravitational interactions (short of mergers) between galaxies. We compare the environments of a sample of nearby (z∼0.15) disc-dominated AGN host galaxies, which have no recent history of mergers (the ‘merger-free’ sample), to a control sample of AGN host galaxies at similar redshifts. We employ three main methods: cylinder searches for potential companion galaxies to the sample galaxies, a search for companion galaxies to the sample galaxies in a well-studied group catalogue and calculation of environment coefficients for each of the galaxies in each sample. We found no significant difference between the environments of galaxies each sample from our cylinder searches with depths of 500 kpc (0.6σ) and 1 Mpc (0.1σ) and only a marginally significant difference between the environments of galaxies from our cylinder search with a depth of 5 Mpc (2.1σ), with the merger-free sample having an increased number of potential companion galaxies found at projected distances close to 500 kpc compared to projected distances ≤ 350 kpc. This effect was not seen as prominently in the control sample. These results may be interpreted to suggest that whilst AGN host galaxies with no recent history of mergers may have less nearby companion galaxies than control AGN host galaxies, they may have a greater number of galaxies gravitationally interacting with them at greater projected distances and with high peculiar velocities. We suggest that SMBH growth in galaxies with no recent history of mergers may be influenced by passing galaxies with high peculiar velocities, which gravitationally interact with galaxies, but do not become gravitationally bound to them. Such interactions may provide enough kinetic energy to gas in the centre of galaxies to fuel SMBH growth. We found no significant difference (1.1σ) between the environments of galaxies in each sample from our analysis of the results of a well-studied group catalogue. This suggests there may be no difference between the environments of bulgeless AGN host galaxies and control AGN host galaxies.Our calculation of environment coefficients for each of the galaxies in each sample found a marginally significant (2.6σ) difference between the environments of each sample. Overall, some of our results suggest that AGN host galaxies with no recent history of mergers may reside in denser environments than control AGN host galaxies, but further work would be required to ascertain whether or not these results are caused by a true difference in environment or by factors such as fibre collisions or redshift differences between samples.