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Title: Revealing powerful obscured AGN through the X-ray background
Author: Gandhi, P.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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The identity of the source of the cosmic X-ray background (XRB) radiation above a few keV has been a mystery for the past 40 years. After the cosmic microwave background was found to have a spectral shape very close to that of a blackbody, it came to be accepted that some kind of discrete sources must produce the bulk of the X-ray background. However, by the mid-1990s, only about 30 per cent of this radiation had been resolved into discrete active galactic nuclei (AGN). The new generation of X-ray telescopes (Chandra and XMM-Newton) have now detected sources which possess the requisite flux and spectral shape to account for the bulk of the background at least up to 10 keV. This thesis presents several sets of observations and an XRB synthesis model, all of which aim to investigate the nature of the source populations which contribute a large fraction of the observed background flux and thus a large fraction of the Universal accretion energy density. An X-ray sample biased towards such sources is compiled from the fields of Chandra observations of galaxy clusters and powerful radio galaxies. These serendipitously-discovered sources cover the flux range ~ 10-15- 10-13 erg s-1 cm-2 [0.5-7 keV]. Starting with the field of Abell 2390, deep optical spectroscopy was carried out for a sample of these serendipitous sources. In this field itself, we find clear identifications of powerful and highly obscured AGN, at least one of which is a member of the class of 'type 2' quasars that should exist in the framework of AGN unification but have remained elusive so far. A sample of hard X-ray sources with weak optical fluxes in a number of fields is then compiled with the aim of discovering more such powerful, obscured sources. Near-infrared spectroscopy reveals a number of sources to have significantly featureless continua consistent with line emission being depleted due to large columns of intrinsic obscuring gas and associated dust. Near-infrared photometry is combined with medium-deep optical photometry to estimate photometric redshifts. The results are broadly consistent with targeted deep-field surveys which find a large fraction of obscured sources at moderate redshifts that are lower than previously predicted. The AGN we target are consistent with being hosted in massive galaxies at a large range of redshifts which peaks close to z = 1. We detect Iron reflection features in a few X-ray spectra. A new background synthesis model which successfully accounts for the recently updated observational constraints (including the areal number density of sources at faint fluxes and the peak of the observed redshift distribution) is then presented and predictions are made regarding observable features on the XRB spectrum. Concluding remarks draw together the results and discuss implications for AGN studies, including future work.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available