Supermassive Black Holes in Disk Galaxies

Dynamical searches find central dark objects – candidate supermassive black holes (BHs) – in at least 34 galaxies. The demographics of these objects lead to the following conclusions: (1) BH mass correlates with the luminosity of the bulge component of the host galaxy, albeit with considerable scatter. The median BH mass fraction is 0.15% of the mass of the bulge. The quartiles are 0.09% and 0.7%. (2) BH mass correlates with the mean velocity dispersion of the bulge component inside its effective radius, i. e., with how strongly the bulge stars are gravitationally bound to each other. For the best BH mass determinations, the scatter is consistent with the measurement errors. (3) BH mass correlates with the luminosity of the high-density central component in disk galaxies independent of whether this is a real bulge (a mini-elliptical) or a “pseudobulge” (believed to form via inward transport of disk material). (4) BH mass does not correlate with the luminosity of galaxy disks. If pure disks contain BHs (and AGN observations argue that some do), then their BH mass fractions are much smaller than the canonical 0.15% for bulges. These results lead to the following conclusions: (A) Present observations show no dependence of BH mass on the details of whether BH feeding happens rapidly during a collapse or slowly via secular evolution of the disk. (B) The above results 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 as an AGN – were the same events.