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
Abstract
We have determined the cosmological evolution of the density of 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~1043 erg s-1) AGN peak at z~0.7, while high luminosity AGN (Lx>1045 erg 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) 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 10-15<S(2-10)<10-10 erg s-1 cm-2 range. We find that the absorbed, high luminosity (Lx>1044 erg s-1) AGN have a density of 267 deg-2 at fluxes S(2-10)>10-15 erg s-1 cm-2. Using these results, we estimate a density of supermassive black holes in the local Universe of 3.2 (h270) x 105 Msol Mpc-3, which is consistent with the recent measurements of the black hole mass function in the local galaxies.
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