Mathematics – Logic
Scientific paper
Oct 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........23s&link_type=abstract
Thesis (PhD). THE UNIVERSITY OF WISCONSIN - MADISON, Source DAI-B 65/04, p. 1910, Oct 2004, 88 pages.
Mathematics
Logic
Scientific paper
To investigate the evolution of Active Galactic Nuclei (AGNs), I conducted a wide-field, moderately deep X-ray survey with the Chandra X-ray Observatory. This survey covers a large, local cosmological volume and samples sources with low surface densities, such as high-luminosity or low-redshift AGNs, while remaining sensitive enough to detect sources that contribute the most to the X-ray background. I present B, Y, R, I, and z' photometry for 521 of the 525 sources in the X-ray catalog and spectroscopic redshifts for 232 (including 17 stars). I do not find evidence for large redshift groupings at z < 1, unlike the apparent excesses found in the Chandra Deep Field surveys. I separate the X-ray sources by optical spectral type and examine the colors, apparent and absolute magnitudes, and redshift distributions for the broad-line and non-broad-line AGNs. I find a definite lack of high- luminosity AGNs at z < 1, consistent with previous observations which showed that supermassive black hole growth is dominated at low redshifts by sources with low accretion rates. I combine my survey with deep Chandra and XMM-Newton X-ray surveys to investigate how the fraction of broad-line sources in the AGN population changes with X-ray luminosity and redshift. From the rest-frame hard-energy (2 8 keV) X-ray luminosity function (HXLF) at z = 0.1 1, I find that broad-line AGNs dominate above ˜3 × 1043 ergs s-1 and have a mean X-ray luminosity of 1.3 × 1044 ergs s-1 . Type II AGNs can only become an important component of the X-ray population at Seyfert-like luminosities. From the z = 0.1 0.5 and z = 0.5 1 HXLFs, I find that the number density of Lx > 10 44 ergs s-1 sources (quasars) steadily declines with decreasing redshift, while the number density of Lx = 10 43 1044 ergs s-1 sources peaks at z = 0.5 1. Strikingly, however, I find that the number densities of broad-line AGNs remain roughly constant with redshift while their average luminosities decline at the lower redshifts. If the accretion that powers AGNs is merger driven, then the decreasing AGN luminosity may be indicative of a change in the merger rate with redshift. An alternative possibility is that the gas fuel supplies are declining at lower redshifts.
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