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

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 Sérsic index of the host spheroidal stellar system and (ii) the measured (spheroid) Sérsic 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 Sérsic 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 parametrized the supermassive black hole mass function with a Schechter function and find, at the low-mass end, a logarithmic slope (1 +α) of ∼0.7 for the full galaxy sample and ∼1.0 for the early-type galaxy sample. Considering spheroidal stellar systems brighter than MB=−18 mag , and integrating down to black hole masses of 106 M⊙ , we find that the local mass density of supermassive black holes in early-type galaxies ρbh,early_type= (3.5 ± 1.2) × 105 h370 M⊙ Mpc−3 , and in late-type galaxies ρbh,late_type= (1.0 ± 0.5) × 105 h370 M⊙ Mpc−3 . The uncertainties are derived from Monte Carlo simulations which include uncertainties in the Mbh–n relation, the catalogue of Sérsic indices, the galaxy weights and Malmquist bias. The combined, cosmological, supermassive black hole mass density is thus Ωbh,total= (3.2 ± 1.2) × 10−6 h70. That is, using a new and independent method, we conclude that (0.007 ± 0.003) h370 per cent of the Universe's baryons are presently locked up in supermassive black holes at the centres of galaxies.