Monsters at the Heart of Galaxy Formation

Abstract

Surveys with the Hubble Space Telescope find supermassive black holes (BHs) of 10**6 to 10**9.5 solar masses in every galaxy that has an elliptical-galaxy-like "bulge" component. BHs appear to be standard equipment in galaxy bulges. At the 2000 June meeting of the American Astronomical Society, two groups announced the discovery of a tight correlation between BH mass and the global velocity dispersion of stars in the host galaxy bulge (astro-ph/0006053 and astro-ph/0006289). Also, BH mass scales with bulge mass; the median BH mass fraction is 0.15 % of the bulge mass. In contrast, BH mass does not scale with disk properties. Pure disk galaxies - ones that lack a bulge component - must have BH mass fractions that are much smaller than the BH mass fractions in bulges. These results imply a close connection between BH growth and bulge (but not disk) formation. Observations increasingly support the hypothesis that the major events that form a bulge or elliptical galaxy and the main growth phases of its BH - when it shone like a quasar - were the same events. These and other results are leading to a profound change in how astronomers view BHs. More than just exotica needed to explain rare active galactic nuclei, they are becoming an integral part of our understanding of galaxy formation.