Local supermassive black holes and relics of active galactic nuclei

We quantify the importance of mass accretion during AGN phases in the growth of supermassive black holes (BH) 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. The density of BHs in the local universe is rho(BH) = 4.6(-1.4)(+1 9) h(0.7)(2) x 10(5) M-circle dot 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 anti-hierarchical in the sense that smaller 13Hs (M-BH < 10(7) M-circle dot) grow at lower redshifts (z < 1) with respect to more massive ones (z similar to 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 from 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 black holes grew during AGN phases in which accreting matter was converted into radiation with efficiencies epsilon = 0.04 - 0.16 and emitted at a fraction lambda = 0.1 - 1.7 and emitted at a fraction lambda = 0.1 - 1.7 of the Eddington luminosity. The average total lifetime of these active phases ranges from similar or equal to 4.5 x 10(8) yr for M-BH < 10(8) M-circle dot to similar or equal to 1.5 x 10(8) yr for M-BH > 10(9) M-circle dot.