Evolution of X-Ray Selected AGN

Abstract

Deep X-ray surveys have shown that the cosmic X-ray background (XRB) is largely due to the accretion onto supermassive black holes, integrated over the cosmic time. These surveys have resolved more than 90% of the X-ray background at ~ 1 keV and about 50% at 10 keV into discrete sources. Optical spectroscopic identifications show that the sources producing the bulk of the X-ray background are a mixture of unobscured (type-1) and obscured (type-2) AGNs, as predicted by the XRB population synthesis models. A class of highly luminous type-2 AGN, so called QSO-2s, has been detected in the deepest Chandra and XMM-Newton surveys. The fraction of type-2 AGN among all AGN, however, decreases significantly with luminosity. The new Chandra AGN redshift distribution peaks at much lower redshifts (z~ 0.7) than that based on ROSAT data. The low redshift peak applies both to absorbed and unabsorbed AGN and is also seen in the 0.5–2 keV band alone. The new, preliminary X-ray luminosity function changes shape between low and high redshifts, confirming the luminosity-dependent density evolution model. The space density of Seyfert galaxies evolves much slower than that of QSOs.

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