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

Abstract. We summarize a study where we test the hypothesis that local black holes (BH) are relics of AGN activity. We compare the mass function of BHs in the local universe with that expected from AGN relics, which are BHs 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. The density of BHs in the local universe is ρBH = 4.6 +1.9 −1.4 h20.7 × 10 5 M ⊙ Mpc −3. The relic BHMF is derived from the continuity equation with the only assumption that AGN activity is due to accretion onto massive BHs and that merging is…