Rapidly spinning massive black holes in active galactic nuclei: evidence from the black hole mass function

The comparison of the black hole mass function (BHMF) of active galactic nuclei (AGN) relics with the measured mass function of the massive black holes in galaxies provides strong evidence for the growth of massive black holes being dominated by mass accretion. We derive the Eddington ratio distributions as functions of black hole mass and redshift from a large AGN sample with measured Eddington ratios given by Kollmeier et al. We find that, even at the low mass end, most black holes are accreting at Eddington ratio λ ∼ 0.2, which implies that the objects accreting at extremely high rates should be rare or such phases are very short. Using the derived Eddington ratios, we explore the cosmological evolution of massive black holes with an AGN bolometric luminosity function (LF). It is found that the resulted BHMF of AGN relics is unable to match the measured local BHMF of galaxies for any value of (constant) radiative efficiency ηrad. Motivated by Volonteri, Sikora & Lasota’s study on the spin evolution of massive black holes, we assume the radiative efficiency to be dependent of black hole mass, i.e., ηrad is low for Mbh < 108M⊙ and it increases with black hole mass for Mbh > 108M⊙. We find that the BHMF of AGN relics can roughly reproduce the local BHMF of galaxies if ηrad ≃ 0.08 for Mbh < 108M⊙ and it increases to & 0.18 for Mbh & 10 9 M⊙, which implies that most massive black holes (& 109M⊙) are spinning very rapidly.