The Hellas2xmm Survey. Vii. the Hard X–ray Luminosity Function of Agn up to Z=4: More Absorbed Agn at Low Luminosities and High Redshifts

We have determined the cosmological evolution of the density of active galactic nuclei (AGN) and of their NH distribution as a function of the un–absorbed 2–10 keV luminosity up to redshift 4. We used the HELLAS2XMM sample combined with other published catalogs, yielding a total of 508 AGN. Our best fit is obtained with a luminosity-dependent density evolution (LDDE) model where low luminosity (LX∼10 43 erg s) AGN peak at z∼0.7, while high luminosity AGN (LX>10 45 erg s) peak at z∼2.0. A pure luminosity evolution model (PLE) can instead be rejected. There is evidence that the fraction of absorbed (NH>10 22 cm) AGN 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 AGN as a function of the flux in the 10 10 44 erg s) AGN have a density of 267 deg at fluxes S2−10>10 −15 erg cm s. Using these results, we estimate a density of supermassive black holes in the local Universe of ρBH = 3.2 h 2 70× 10 5 M⊙ Mpc , which is consistent with the recent measurements of the black hole mass function in the local galaxies. Subject headings: diffuse radiation — galaxies: active — galaxies: evolution — quasars: general — surveys — X–rays: diffuse background