The relationship between obscured AGN and their host galaxies
File(s)
Thesis
Author(s)
Rangel, Cyprian
Type
Thesis
Abstract
This thesis presents an investigation into obscured Active Galactic Nuclei (AGN) activity at high redshift and the interactions they share with their host galaxies. Using a combination of X-ray spectral fitting and X-ray stacking analyses, three studies are undertaken in this work. The first study is a reinvestigation of a specific group of X-ray undetected mid-IR excess galaxies at z ~ 2 that have previously been identified as Compton thick AGN candidates through X-ray stacking analysis. The parent sample of optically identified galaxies is found to possess above average obscured AGN activity. The galaxies exhibiting mid-IR excess, however, do not exhibit elevated levels of obscured AGN activity relative to the parent sample. Key to this result is the increased depth of X-ray observations, which resolves hard X-ray sources that had biased earlier stacking analyses.
The second study concerns the nature of AGN residing in massive galaxies at z ~ 2. The highlight of this research is the identification of two accretion modes which are dependent upon host galaxy compactness: a “transformative mode for compact galaxies and a “maintenance mode for extended galaxies. AGN in the transformative mode are heavily obscured and X-ray luminous and are thought to rapidly quench star formation in their host galaxies through violent feedback. The AGN in “maintenance mode have lower luminosities and tend to be unobscured, but appear to suppress further star formation in their host galaxies through a gentler feedback process.
The third study is a comparison of colour-excitation (CEx) and mass-excitation (MEx) classification techniques designed to identify Type 2 AGN out to z ~ 0.8. The CEx technique is found to identify obscured AGN with a high degree of accuracy, with X-ray stacking revealing many as yet X-ray undetected sources. The MEx technique is adept at identifying X-ray detected AGN but is less accurate at isolating obscured sources.
The second study concerns the nature of AGN residing in massive galaxies at z ~ 2. The highlight of this research is the identification of two accretion modes which are dependent upon host galaxy compactness: a “transformative mode for compact galaxies and a “maintenance mode for extended galaxies. AGN in the transformative mode are heavily obscured and X-ray luminous and are thought to rapidly quench star formation in their host galaxies through violent feedback. The AGN in “maintenance mode have lower luminosities and tend to be unobscured, but appear to suppress further star formation in their host galaxies through a gentler feedback process.
The third study is a comparison of colour-excitation (CEx) and mass-excitation (MEx) classification techniques designed to identify Type 2 AGN out to z ~ 0.8. The CEx technique is found to identify obscured AGN with a high degree of accuracy, with X-ray stacking revealing many as yet X-ray undetected sources. The MEx technique is adept at identifying X-ray detected AGN but is less accurate at isolating obscured sources.
Version
Open Access
Date Issued
2013-09
Date Awarded
2014-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Nandra, Kirpal
Warren, Stephen
Sponsor
Science and Technology Facilities Council (Great Britain)
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)