6 v 1 5 S ep 2 00 0 Black Hole Demographics from the M • − σ Relation

We analyze a sample of 32 galaxies for which a dynamical estimate of the mass of the hot stellar component, M bulge , 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 • /M bulge)] is reasonably well described as a Gaussian with log(M • /M bulge) ≈ −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 × 10 5 M ⊙ Mpc −3 , consistent with that inferred from high redshift (z ∼ 2) AGNs.