Black hole demographics from the M•[BH] − σ[sigma] relation

Abstract

We analyze 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 the 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 to bulge mass is a factor ~5 smaller than generally quoted in the literature. We comment on marginal evidence for a lower, average black-hole mass fraction in more massive galaxies, which should be investigated using larger samples. The total mass density in BHs in the local Universe is estimated to be ~5×105M⊙Mpc−3, consistent with that inferred from high redshift (z~2) AGNs. (Refer to PDF file for exact formulas).