Supermassive black holes are thought to be relics of quasars, and their numbers and masses are therefore related to the quasar luminosity function and its evolution with redshift. We have used the relationship between black hole mass and bulge velocity dispersion (the M • − σ relation) to make an improved estimate of the mass density and mass spectrum of supermassive black holes. Uncertainties in the M • − σ relation have little effect on the mass density. We find a mass density of (4.8 ± 1.6)h 2 · 10 5 M⊙Mpc −3. Some of the variance in published density estimates comes from the use of different values of the Hubble constant. Subject headings: galaxies: general, nuclei, black holes 1
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