Investigating the Relationship between Environment and Active Galactic Nuclei activity at High Redshift
Author(s)
Digby-North, Jonathan A
Type
Thesis
Abstract
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.
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.
Date Issued
2011
Date Awarded
2011-12
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Nandra, Kirpal
Jaffe, Andrew
Creator
Digby-North, Jonathan A
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)