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dc.contributor.authorDowsett, Rachelen
dc.date.accessioned2018-01-31T11:42:50Z
dc.date.available2018-01-31T11:42:50Z
dc.date.issued2006en
dc.identifier.urihttp://hdl.handle.net/1842/27937
dc.description.abstracten
dc.description.abstractIn 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.en
dc.description.abstractI 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.en
dc.description.abstractIn 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.en
dc.description.abstractIn 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.en
dc.description.abstractUsing 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.en
dc.publisherThe University of Edinburghen
dc.relation.isreferencedbyen
dc.subjectAnnexe Thesis Digitisation Project 2017 Block 16en
dc.titleUnderstanding the link between active galactic nuclei and their large scale environmenten
dc.typeThesis or Dissertationen
dc.type.qualificationlevelDoctoralen
dc.type.qualificationnamePhD Doctor of Philosophyen


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