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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 Mbullet - sigma relation) to make an improved estimate of the mass density and mass spectrum of supermassive black holes. Uncertainties in the Mbullet - sigma relation have little effect on the mass density. We find a mass density of (4. 8 +/- 1. 6) h² x 10⁵ Mₛun Mpc^-3. Some of the variance in published density estimates comes from the use of different values of the Hubble constant.
Aller et al. (Sun,) studied this question.