Supermassive Black Holes in Galactic Nuclei: Co-Evolution (Or Not) of Black Holes and Host Galaxies

Stellar- and gas-dynamic searches reveal 106 to 109.5 solar mass "supermassive black holes" (BHs) in 30 elliptical and 38 disk galaxies.  The main demographic result is the observation of tight correlations between BH masses and host galaxies which imply that BH growth and galaxy formation regulate each other.  The "low-hanging fruit" on this co-evolution is to improve our understanding of host galaxies.  The result is this: BHs correlate only with classical bulges, i.e. with the remnants of mergers of similar-mass galaxies. They do not correlate with galaxy disks or with bulge-like, high-density central components that were grown slowly out of galaxy disks. Absent bulges, they also do not correlate with dark matter halos of galaxies.  BHs exist in some bulgeless galaxies but do not correlate with them.  We suggest that there are two modes of BH growth.  Black holes in bulges grow rapidly to high masses when mergers drive gas infall that feeds quasar-like events. Such global processes result in co-evolution.  In contrast, BHs in bulgeless galaxies are fed only locally near galaxy centers and so do not co-evolve with their host galaxy disks.