  We provide a new estimate of the local supermassive black hole mass function
using (i) the empirical relation between supermassive black hole mass and the
Sersic index of the host spheroidal stellar system and (ii) the measured
(spheroid) Sersic indices drawn from 10k galaxies in the Millennium Galaxy
Catalogue. The observational simplicity of our approach, and the direct
measurements of the black hole predictor quantity, i.e. the Sersic index, for
both elliptical galaxies and the bulges of disc galaxies makes it
straightforward to estimate accurate black hole masses in early- and late-type
galaxies alike. We have parameterised the supermassive black hole mass function
with 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 galaxy
sample. Considering spheroidal stellar systems brighter than M_B = -18 mag, and
integrating down to black hole masses of 10^6 M_sun, we find that the local
mass 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-type
galaxies rho_{bh, late-type} = (1.0+/-0.5) x 10^5 h^3_{70} M_sun Mpc^{-3}. The
uncertainties are derived from Monte Carlo simulations which include
uncertainties in the M_bh-n relation, the catalogue of Sersic indices, the
galaxy weights and Malmquist bias. The combined, cosmological, supermassive
black 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 in
supermassive black holes at the centres of galaxies.

Astrophysics