Cosmological Evolution of the Hard X-Ray AGN Luminosity Function: Formation History of Supermassive Black Holes

The cosmological evolution of the hard X-ray luminosity function(HXLF) of Active Galactic Nuclei (AGN) is investigated in the 2--10 keV luminosity range of 10^41.5 - 10^46.5 erg s^-1 as a function of redshift up to 3. From a combination of surveys conducted at photon energies above 2 keV with HEAO1, ASCA and Chandra, we construct a highly complete (>96 over the wide flux range of 10^-10 -- 3.8 × 10^-15 erg cm^-2 s^-1 (2--10 keV). We find that (i) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (ii) the evolution of the HXLF of the whole AGNs (including both type-I and type-II AGNs) is best described with a luminosity dependent density evolution where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background, thus solving its origin quantitatively. Based on these results, we discuss the formation history of supermassive black holes in galactic centers. The total accreted mass traced by the HXLF with an accretion mass-to-energy conversion factor of 0.1 is consistent with an estimate of SMBH mass density from the demography of galaxies in nearby universe.