The X-ray Background (Deep Fields, Luminosity Functions and Type-Ⅱ Quasars)

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 80% of the 0.1-10keV X-ray background into discrete sources.Optical spectroscopic identifications show that the sources producing the bulk of the X-ray background are a mixture of obscured (type-1)and unobscured (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 new Chandra AGN redshift distribution peaks at much lower redshifts (z≈0.7)than that based on ROSAT data, indicating that Seyfert galaxies peak at significantly lower redshifts than QSOs.