In this thesis I present an analysis of the population of AGN in dense environments, with
the aim of determining the number of AGN in galaxy clusters and investigating the effect of
extended environment on the AGN. This work uses the new generation of X-ray telescopes
to identify and analyse the populations of AGN, specifically focusing on clusters of galaxies.
I have undertaken the first major statistical survey to investigate the prevalence of AGN in a
large sample of moderate redshift (z > 0.1) galaxy clusters. By comparing the X-ray source
population in the fields of 140 galaxy clusters to that found in 44 non-cluster observations,
the number of X-ray detected AGN associated with each galaxy cluster is determined statistically. In order to analyse the large number of observations in a fast and uniform manner, I
have developed an automated pipeline which detects and evaluates the X-ray point sources
in each field. The pipeline also produces a prediction of the number density and radial distribution of sources in each field, based on the non-cluster observations and detailed models
of the detector characteristics. The effect of gravitational lensing, which reduces the X-ray
source counts by up to 1 source per cluster field, is calculated as a function of radius, and
incorporated into the predicted source distribution. I demonstrate that there is a significant
population of AGN in galaxy clusters, with an average of ~ 3 AGN per cluster.
In order to evaluate the effect of different cluster environments on the AGN population, and
the evolution of cluster AGN, I investigate trends in the excess of point sources as a function
of cluster redshift, luminosity and morphology. It is found that low redshift (z < 0.5) clusters contain almost no AGN in their central regions, but that a moderate number of AGN are
found between 0.5 and 1 Mpc from the cluster centre. By comparison with X-ray sources in
the field it is determined that AGN are suppressed in the centres of galaxy clusters, and it is
likely that they are triggered on the outskirts of clusters. At higher redshifts there is a significant increase in the number of luminous AGN in galaxy clusters, which is greater than the
evolution of field AGN, indicating that the evolution of the AGN population is dependent
on the extended environment. The number of AGN in galaxy clusters is found to increase
in disturbed clusters, and it appears that the AGN are affected by local galaxy density rather
than the properties of the galaxy cluster.
In addition to the large statistical study, I have investigated in detail the number and properties of AGN in the supercluster A901/2 (z=0.17). This supercluster is a highly complex
environment, which is highlighted by analysis of the X-ray extended emission. I describe a
previously unidentified cluster, A901o;, and the properties of extended emission from four
other clusters and groups in this region. Using deep X-ray imaging, 17-band photometric
data and optical spectroscopy, I identify 11 AGN in the supercluster. Around 5% of bright
( r r i R < 20) supercluster galaxies are found to contain an AGN at this flux limit - for the
most part these appear optically to be passive early-type galaxies, and there are significantly
more AGN than would be expected from the optical data alone.
Using the positions and colours of over 1000 identified supercluster galaxies, I define the
supercluster environments in terms of local galaxy density and local galaxy colour. Compared to other similar galaxies, AGN host galaxies are found to lie in areas of moderate
density and slightly higher density with blue local colour (but not in redder environments
of the same density), similar to that of groups of galaxies and cluster outskirts. A possible explanation is that AGN activity is triggered by tidal disruption or harassment when a
galaxy first joins a dense environment. There is also tentative evidence for a lower X-ray
luminosity in AGN in the most cluster-like environments.
