The Star-Formation Properties of Nearby AGN in the Observed and Simulated Universe: BAT vs EAGLE

Abstract

In this thesis we present data from 272 low redshift, hard X-ray (14 - 195 keV) selected Active Galactic Nuclei (AGN) taken from the Swift-BAT all sky survey. We calculated Star Formation Rates (SFRs) and Specific Star Formation Rates (sSFRs) via Spectral Energy Distribution fitting in the optical and infra red parts of the elctromagnetic spectrum. The host galaxy properties of a sub-sample (72 AGN) were compared to a flux and volume matched sample of AGN from the EAGLE hydrodynamical simulations in order to verify how accurately the simulations can reproduce the observational data, including some Monte-Carlo methods. We find Anderson-Darling test agreements in the SFR distributions of 20 (45 in the MC methods), however < 1 in the sSFR distributions. This difference is driven by an on average underprection of the sSFR per stellar mass bin in EAGLE. The X-ray luminosity distributions are also poorly reproduced (1.8 agreement). We postulate that the difference in X-ray luminosities is due to the short timescales on which AGN activity varies and that general differences may be accounted for due to the compton thick population. We find that the median of the sSFR distributions of both our observed and simulated AGN lie below the sSFR main sequence, in agreement with previous studies. We also explore the evolution of average AGN host galaxy properties within EAGLE in high time resolution data (1 Myr steps) over the last 700 Myr and throughout cosmic time up to in low time resolution steps (21 snapshots). This firstly reveals factors in EAGLE such as critical masses (M 10 M) at which AGN feedback starts to have a significant effect on the host galaxy, mainly via the quenching star formation. It secondly reveals the effect of quasar-like events (L 10 erg s) which can significantly quench star formation. We also investigate the cosmic evolution of average host galaxy properties of two differently selected populations of AGN in EAGLE. We find that galaxies selected on similar criteria from different epochs show similar behaviours but display different average host galaxy properties in the present day.