AGN Accretion History revealed from X-ray surveys

G. Szokoly

New results on the cosmological evolution of unabsorbed (type-1) active galactic nuclei (AGN) selected in the soft (0.5-2 keV) X--ray band are presented. From a variety of ROSAT, XMM-Newton and Chandra surveys we selected a total of more than 1000 AGN with an unprecedented spectroscopic and photometric optical/NIR identification completeness. For the first time we are able to derive reliable space densities for low-luminosity (Seyfert--type) X-ray sources at cosmological redshifts. The evolutionary behaviour of AGN shows a strong dependence on X-ray luminosity: while the evolution of the space density of high--luminosity AGN saturates at redhifts above $z\sim 1.5$, similar to the optically selected QSO samples, the space density of low--luminosity AGNs peaks at redshifts below z=1. This confirms previous ROSAT findings of a luminosity-dependent density evolution. Using a rigorous treatment of the optical identification completeness we are able to show that the space density of AGN with X-ray luminosities L_X < 10^{44} erg/s declines significantly towards high redshifts. At higher luminosities (QSOs), the statistics are still too poor to constrain the putative decline in space density. In conclusion I will discuss ongoing and planned new, wider field X-ray surveys, which should yield large enough numbers of high-redshift QSO to determine their high-redshift evolution.