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Supermassive black holes are thought to be relics of quasars, and theirnumbers and masses are therefore related to the quasar luminosity function andits evolution with redshift. We have used the relationship between black holemass and bulge velocity dispersion (the M_bullet - sigma relation) to make animproved estimate of the mass density and mass spectrum of supermassive blackholes. Uncertainties in the M_bullet - sigma relation have little effect on themass density. We find a mass density of (4.8 +/- 1.6) h^2 x 10^5 M_sun Mpc^-3.Some of the variance in published density estimates comes from the use ofdifferent values of the Hubble constant.Comment: To appear in the December 2002 issue of The Astronomical Journa

The Cosmic Density of Massive Black Holes from Galaxy Velocity Dispersions