The HELLAS2XMM Survey. VII. The Hard X-Ray Luminosity Function of AGNs up to z = 4: More Absorbed AGNs at Low Luminosities and High Redshifts

We have determined the cosmological evolution of the density of active galactic nuclei (AGNs) and of their N_H distribution 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^-1) AGNs peak at z ~ 0.7, while high-luminosity AGNs (L_X > 10⁴⁵ ergs s^-1) 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 cm^-2) 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 cm^-2 s^-1 < S_2-10 < 10^-10 ergs cm^-2 s^-1 range. We find that the absorbed, high-luminosity (L_X > 10⁴⁴ ergs s^-1) AGNs have a density of 267 deg^-2 at fluxes S_2-10 > 10^-15 ergs cm^-2 s^-1. Using these results, we estimate a density of supermassive black holes in the local universe of ρ_BH = 3.2 h × 10⁵ M_☉ Mpc^-3, which is consistent with the recent measurements of the black hole mass function in the local galaxies.