The demography of massive galactic black holes

Abstract

This review describes dynamical evidence for massive black holes (BHs) at the centers of galaxies. BHs appear to be a common if not ubiquitous feature of local galactic spheroids, with a mean mass that is roughly 0.5% of the parent stellar spheroid mass. This mass is large enough that the formation and evolution of central BHs has had a significant influence on the inner stellar density profile. For example, low-density, core-type profiles may have been created by the scouring action of binary BH pairs, whose orbital decay ejects stars from the center of the galaxy. Or, BHs may have grown substantially by gas accretion, pulling in surrounding stars and creating the cuspy cores and power laws that are seen in spheroids. If BHs are ubiquitous in galactic spheroids at the above mass ratio, their total local mass density is comparable to that implied by the energy density of QSO photons, suggesting that these local BHs are the long-sought fossil BHs of QSO central engines. However, QSOs are much rarer per unit co-moving volume than local BHs, unless one equates them to just the massive BHs found in rare and massive local spheroids. The resultant BH masses exceed typical QSO BH mass estimates by nearly a factor of 10, suggesting that BHs have grown in mass by roughly that factor since the QSO era. At least some of that growth might have occurred as protogalaxies grew in mass by mergers, and their BHs also merged and grew apace.