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 NH 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 (LX ~ 1043 ergs s-1) AGNs peak at z ~ 0.7, while high-luminosity AGNs (LX > 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 (NH > 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 < S2-10 < 10-10 ergs cm-2 s-1 range. We find that the absorbed, high-luminosity (LX > 1044 ergs s-1) AGNs have a density of 267 deg-2 at fluxes S2-10 > 10-15 ergs cm-2 s-1. Using these results, we estimate a density of supermassive black holes in the local universe of ρBH = 3.2 h × 105 M☉ Mpc-3, which is consistent with the recent measurements of the black hole mass function in the local galaxies.