Characterising the X-ray selected AGN Content of the Nearby Universe

Large-scale surveys of the Active Galactic Nuclei (AGN) population performed over the past few decades have highlighted the existence of strong correlations between the central supermassive black hole and their host galaxy. Whilst such samples are incredibly useful, they tend to be incomplete. This skews our understanding of the AGN population as samples consist of largely brighter, more actively accreting objects. In this thesis, I combined SDSS galaxy samples with XMM-Newton serendipitous sources to measure the incidence of AGN as a function of galaxy properties in the nearby (z < 0:35) Universe, carefully accounting for the incompleteness of the X-ray imaging. Regardless of how the galaxy population were split, the probability of hosting an AGN was well described by a power law and highlighted significant amounts of activity at low X-ray luminosities and accretion rates. I identified 61 AGN in dwarf galaxies, of which 40 have not been previously identified. The probability of hosting an AGN in this regime was unaffected by either the stellar mass or redshift of the host galaxy. Then I studied the wider local galaxy population. As with dwarf galaxies, the probability of hosting an AGN is not affected by stellar mass but it does increases with redshift. When comparing my results to higher redshift measurements, I identified a stellar-mass-dependent distribution of black hole growth rates. Finally, I investigated the effect of star-forming activity. I found that the probability of a galaxy hosting an AGN increases with the star formation rate. Throughout, I also compared the effectiveness of AGN selection at X-ray and optical wavelengths. I found that the BPT diagnostic is not as effective as X-ray selection. The optical diagnostic incorrectly classified a significant proportion of the X-ray selected AGN sample as star-forming, and was unable to identify weak AGN emission.