The Masses of Nuclear Black Holes in Luminous Elliptical Galaxies and Implications for the Space Density of the Most Massive Black Holes

Black hole (BH) masses predicted from the M•-σ relationship conflict with predictions from the M•-L relationship for high-luminosity galaxies, such as brightest cluster galaxies (BCGs). The M•-L relationship predicts that some BCGs may harbor BHs with M• approaching 1010 M☉, while the M•-σ relationship always predicts M• < 3 × 109 M☉. We argue that the M•-L relationship is a plausible description for galaxies of high luminosity. If the cores in central stellar density are formed by binary BHs, the inner core cusp radius, rγ, may be an independent witness of M•. Using central structural parameters derived from a large sample of early-type galaxies observed by HST, we argue that L is superior to σ as an indicator of rγ. Further, the rγ-M• relationship for 11 core galaxies with measured M• appears to be consistent with the M•-L relationship for BCGs. BCGs have large cores appropriate for their large luminosities that may be difficult to generate with the more modest BH masses inferred from the M•-σ relationship. M• ~ M would be expected for BCGs, if they were formed in dissipationless mergers, which should preserve the ratio of BH to stellar mass, M. This scenario appears to be consistent with the slow increase in σ with L and the more rapid increase in effective radii with L seen in BCGs as compared to less luminous galaxies. If BCGs have large BHs commensurate with their luminosities, then the local BH mass function for M• > 3 × 109 M☉ would be nearly an order of magnitude richer than that inferred from the M•-σ relationship. The volume density of the most luminous QSOs may favor the M•-L relationship.