Black hole demographics from the M• - σ relation

Abstract

We analyse a sample of 32 galaxies for which a dynamical estimate of the mass of the hot stellar component, Mbulge, is available. For each of these galaxies, we calculate the mass of the central black hole, M•, using the tight empirical correlation between M• and bulge stellar velocity dispersion. The frequency function N[log(M•/Mbulge)] is reasonably well described as a Gaussian with 〈log(M•/Mbulge)〉 ≈ -2.90 and standard deviation ∼0.45; the implied mean ratio of black hole mass to bulge mass is a factor of ∼5 smaller than generally quoted in the literature. We present marginal evidence for a lower, average black hole mass fraction in more massive galaxies. The total mass density in black holes in the local Universe is estimated to be ∼5 × 105 M⊙Mpc-3, consistent with that inferred from high-redshift (z ∼ 2) active galactic nuclei.