Supermassive black holes in galactic nuclei

As an example of how the Hubble Space Telescope (HST) has strengthened dynamical arguments for supermassive black holes (BHs), we review the case of M 31. The M-. similar or equal to 3 x 10(7) Me central massive dark object (MDO), discovered using spectra of 1" resolution, has been observed at 10 times better resolution with the Space Telescope Imaging Spectrograph (STIS). The observations show that the MDO is located in a blue cluster of A-type stars embedded in P2, one of the two nuclei of M 31. The cluster's tiny size and large velocity dispersion, sigma greater than or equal to 800 km s(-1) suggest that M-. is confined to a radius r less than or equal to 0."06 = 0.22 pc. If this is confirmed, then M 31 is the third galaxy (after NGC 4258 and our Galaxy) in which dark clusters of brown dwarfs or dead stars can be excluded on astrophysical grounds. This strengthens the inference that dynamically detected MDOs in other galaxies are supermassive black holes also. The demographics of 38 BH detections lead to these 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 0.13% of the mass of the bulge. (2) BH mass correlates with the mean velocity dispersion of the bulge inside its effective radius, i.e., with how strongly the bulge stars are gravitationally bound to each other. (3) BH mass does not correlate with the luminosity of galaxy disks. If pure disks contain BHs, then M-. much less than 0.13% of the mass of the disk. (4) To, first order, BH mass does not depend on whether the high-density central component in disk galaxies is a bulge (a mini-elliptical, believed to form via a merger-induced dissipative collapse and starburst) or a "pseudobulge" (believed to form secularly by inward transport of disk gas). Thus 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. The results also 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 a quasar, were the same events.