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 similar to 10(43) ergs s(-1)) AGNs peak at z similar to 0.7, while high-luminosity AGNs (L-X > 10(45) ergs s(-1)) peak at z similar to 2.0. A pure luminosity evolution model (PLE) can instead be rejected. There is evidence that the fraction of absorbed (N-H > 10(22) 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 (L-X > 10(44) 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 rho(BH) = 3.2 h(70)(2) x 10(5) M-circle dot Mpc(-3), which is consistent with the recent measurements of the black hole mass function in the local galaxies.