T. Dwelly
The demographics and evolution of the absorbed AGN population
2006 (supervisor: M. J. Page)
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.