Active galactic nuclei and the unresolved cosmic X-ray background
Abstract
This thesis presents observational studies of active galactic nuclei (AGNs) and the extragalactic cosmic X-ray background (CXB), with the goal of exploring the nature and cosmic evolution of AGNs and galaxies. This work includes a detailed study of the faintest known X-ray emitting galaxies, the detection of the largest sample to date of luminous, obscured AGNs, and the first study of AGN- galaxy clustering in the AGES redshift survey.
The first part presents studies of the unresolved CXB in the deepest extragalactic X-ray observations, the 1 and 2 Ms Chandra Deep Fields. The spectrum of the CXB is measured after exclusion of all detected X-ray sources and careful subtraction of instrumental background. 77% ± 3% and 80% ± 8% of the CXB is resolved in the 1-2 keV and 2-8 keV bands respectively (smaller than some previous estimates) and the remaining 1-2 keV CXB flux cannot be accounted for by a simple extrapolation of the observed X-ray log N -log S. This indicates the possibility of a new population of faint X-ray sources, which we explore further by measuring the CXB after also excluding Hubble Space Telescope ( HST ) and Spitzer sources from the CDFs. Finally, we constrain the underlying X-ray flux distribution of the X-ray undetected HST galaxies, which are shown to have a log N -log S consistent with an extension of the observed population of faint star-forming galaxies rather than AGNs. The second part focuses on studies of AGNs in the 9 deg 2 multiwavelength survey in Boötes. Using Spitzer mid-infrared observations, we identify 640 luminous, obscured AGN at 0.7 < z < 3, and confirm this selection based on average X-ray properties from the Chandra XBoötes survey. The IR AGNs show a possible bimodality in their extinction, which may place constraints on the accretion geometry (for unified models) or timescales for AGN feedback. We also derive the two-point cross-correlation of AGNs and galaxies at 0.25 < z < 0.8 using redshifts from AGES. Separating X-ray and radio AGNs into those with red and blue host galaxies reveals significant differences in clustering, X-ray spectra, and Eddington ratios, which suggests that these populations may represent different modes of AGN accretion.- Publication:
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Ph.D. Thesis
- Pub Date:
- 2008
- Bibcode:
- 2008PhDT.........4H
- Keywords:
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- Cosmic X-ray background;
- Active galactic nuclei;
- Galaxies;
- X-ray emitting galaxies