2005ApJ...635..864L

We have determined the cosmological evolution of the density of active galactic nuclei (AGNs) and of their N_Hdistribution as a function of the unabsorbed 2-10 keV luminosity up to redshift 4. We used the HELLAS2XMM sample combined with other published catalogs, yielding a total of 508 AGNs. Our best fit is obtained with a luminosity-dependent density evolution (LDDE) model where low-luminosity (L_X∼10⁴³ ergs/s) AGNs peak at z∼0.7, while high-luminosity AGNs (L_X>10⁴⁵ ergs/s) peak at z∼2.0. A pure luminosity evolution model (PLE) can instead be rejected. There is evidence that the fraction of absorbed (N_H>10²²/cm2) AGNs decreases with the intrinsic X-ray luminosity and increases with the redshift. Our best-fit solution provides a good fit to the observed counts, the cosmic X-ray background, and to the observed fraction of absorbed AGNs as a function of the flux in the 10^–15 ergs/cm2/s<S_2–10<10^–10 ergs/cm2/s range. We find that the absorbed, high-luminosity (L_X>10⁴⁴ ergs/s) AGNs have a density of 267/deg2 at fluxes S_2–10>10^–15 ergs/cm2/s. Using these results, we estimate a density of supermassive black holes in the local universe of ρ_BH=3.2 h²₇₀x10⁵ M_☉/Mpc3, which is consistent with the recent measurements of the black hole mass function in the local galaxies.