Local supermassive black holes, relics of active galactic nuclei and the X-ray background

ABSTRACT. We quantify the importance of mass accretion during active galactic nuclei (AGN) phases in the growth of supermassive black holes (BHs) by comparing the mass function of black holes in the Local Universe with that expected from AGN relics, which are black holes grown entirely with mass accretion during AGN phases. The local BH mass function (BHMF) is estimated by applying the well-known correlations between BH mass, bulge luminosity and stellar velocity dispersion to galaxy luminosity and velocity functions. We find that different correlations provide the same BHMF only if they have the same intrinsic dispersion. The density of supermassive black holes in the Local Universe that we estimate is ρBH=…