Understanding the link between active galactic nuclei and their large scale environment

• Main Author: Dowsett, Rachel
• Published: University of Edinburgh 2006
• Subjects: 520
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.649700

I have undertaken the first major statistical survey to investigate the prevalence of AGN in a large sample of moderate redshift (0.1 < z < 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, I determine statistically the number of X-ray detected AGN associated with each galaxy cluster. I demonstrate that there is a significant population of AGN in galaxy clusters. The effect of gravitational lensing reduces the X-ray source counts in cluster fields by up to 1 source per field. I calculate the expected source deficit due to lensing for each cluster, as a function of radius, and incorporate this into the predicted source distribution. I have also investigated in detail the number and properties of AGN in the supercluster A901/2 (z=0.17). Using deep X-ray imaging, 17-band photometric data and optical spectroscopy I identify 11 AGN in the supercluster. Around 5% of bright (<i>M_R </i>< 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. Compared to other similar galaxies, AGN host galaxies are found to lie in areas of moderate density and blue local colour, 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.