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Title: Investigating the relationship between environment and Active Galactic Nuclei activity at high redshift
Author: Digby-North, Jonathan A.
ISNI:       0000 0004 2713 5159
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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This thesis presents an investigation into the relationship between large-scale structure environment and Active Galactic Nuclei (AGN) activity at high redshift. To accomplish this, the environments of AGN are studied from two complementary perspectives. Firstly, various observations of a specific large-scale structure at z = 2.3 are used to assess the level of AGN activity in relation to the field. The main result of this study is that both the emission-line and X-ray selected AGN populations are significantly enhanced; X-ray detected Lyman Alpha Emitters (LAEs), however, are not found to be significantly enhanced. The host galaxy properties of z ~ 1 X-ray sources are then derived and studied in detail. Confirming previous results, X-ray sources are found in optically luminous (MB ~< −20.5mag), massive (log(M∗/ M⊙) ≥ 10.5) red and blue hosts. A larger fraction of red/green hosts harbour obscured (log(NH) ≥ 22) AGN than blue, with the most obscured sources (log(NH) ≥ 23.5) also being more frequently found in red/green host galaxies than blue. The second approach used the 3rd-nearest neighbour measure to study the environment of X-ray hosts at z ~ 1, accounting for their optical colour, luminosity and stellar mass. A main new and important result is that X-ray hosts are found in regions of enhanced density compared to optical galaxies of equivalent mass, which is not due to the observed colour-density relation at z ~ 1. The enhancement is found to be most significant at the reddest colours, brightest luminosities, and highest stellar masses. The results from this thesis show that the dense environments probed in this work generally promote AGN activity. This is probably not due to major mergers, but could be due to an increased probability of minor mergers/interactions and/or milder environmental processes triggering nuclear activity. Alternatively, perhaps there is some other galaxy property (e.g., residing in higher mass haloes) which is conducive to AGN activity.
Supervisor: Nandra, Kirpal ; Jaffe, Andrew Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral