The Millennium Galaxy Catalogue: The local supermassive black hole mass function in early- and late-type galaxies

摘要

We provide a new estimate of the local supermassive black hole mass functionusing (i) the empirical relation between supermassive black hole mass and theSersic index of the host spheroidal stellar system and (ii) the measured(spheroid) Sersic indices drawn from 10k galaxies in the Millennium GalaxyCatalogue. The observational simplicity of our approach, and the directmeasurements of the black hole predictor quantity, i.e. the Sersic index, forboth elliptical galaxies and the bulges of disc galaxies makes itstraightforward to estimate accurate black hole masses in early- and late-typegalaxies alike. We have parameterised the supermassive black hole mass functionwith a Schechter function and find, at the low-mass end, a logarithmic slope(1+alpha) of ~0.7 for the full galaxy sample and ~1.0 for the early-type galaxysample. Considering spheroidal stellar systems brighter than M_B = -18 mag, andintegrating down to black hole masses of 10^6 M_sun, we find that the localmass density of supermassive black holes in early-type galaxies rho_{bh,early-type} = (3.5+/-1.2) x 10^5 h^3_{70} M_sun Mpc^{-3}, and in late-typegalaxies rho_{bh, late-type} = (1.0+/-0.5) x 10^5 h^3_{70} M_sun Mpc^{-3}. Theuncertainties are derived from Monte Carlo simulations which includeuncertainties in the M_bh-n relation, the catalogue of Sersic indices, thegalaxy weights and Malmquist bias. The combined, cosmological, supermassiveblack hole mass density is thus Omega_{bh, total} = (3.2+/-1.2) x 10^{-6} h_70.That is, using a new and independent method, we conclude that (0.007+/-0.003)h^3_{70} per cent of the universe's baryons are presently locked up insupermassive black holes at the centres of galaxies.