The demographics and evolution of the absorbed AGN population

It has become increasingly apparent that active galactic nuclei (AGN) have played a key role in the galaxy formation process, leading to the galaxy population we see today. In order to understand better this inter-relationship, we must first measure the characteristics and evolution of the AGN population over cosmic timescales. Models of the AGN population which reproduce the spectrum and intensity of the extragalactic X-ray background require a large population of faint AGN, the majority obscured by large column densities of cold material. In the local Universe, where we find mostly low luminosity objects, the obscured AGN make up ∼80% of the population. However, at higher redshifts, including the epoch when AGN and galaxies were forming most rapidly, the demographics of the obscured AGN population are still poorly understood. For this thesis, I have made a detailed examination of the AGN detected in several of the deepest XMM-Newton `blank-field' observations. I have carried out a detailed set of Monte-Carlo simulations in order to compare directly the X-ray properties of the observed AGN to the predictions of a number of AGN population models. For sources detected in the `13H' deep field, I find that the best fitting model contains AGN with a broad range of obscuration levels, but with significantly absorbed AGN making up at least 75% of the population. Furthermore, by examining the sources in XMM-Newton observations of the `CDFS' field, for which nearly complete redshift determinations are available, I find that the AGN absorption distribution exhibits little redshift or luminosity dependence. I confirm these findings by extending my study to a much larger AGN sample, and investigate field-to-field AGN source density variations.