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 not important. We find that the relic BHMF at z = 0 is generated mainly at z < 3. Moreover, the BH growth is antihierarchical in the sense that smaller BHs (MBH < 10 7 M⊙) grow at lower redshifts (z < 1) with respect to more massive ones (z ∼ 1 − 3). Unlike previous work, we find that the BHMF of AGN relics is perfectly consistent with the local BHMF indicating the local BHs were mainly grown during AGN activity. This agreement is obtained while satisfying, at the same time, the constraints imposed by the X-ray background. The comparison with the local BHMF also suggests that the merging process is not important in shaping the relic BHMF, at least at low redshifts (z < 3). Our analysis thus suggests the following scenario: local BHs grew during AGN phases in which accreting matter was converted into radiation with efficiencies ε = 0.04 − 0.16 and emitted at a fraction λ = 0.1 − 1.7 of the Eddington luminosity. The average total lifetime of these active phases ranges from ≃ 4.5 × 10 8 yr for MBH < 10 7 M ⊙ to ≃ 1.5 × 10 8 yr for MBH> 10 9 M⊙. 1