HST Detections of Massive Black Holes in the Centers of Galaxies

After correcting spherical aberration in the Hubble Space Telescope in 1993, the central masses of galaxies can be measured with a resolution 5 to 10 times better than can be achieved at the best terrestrial sites. This improvement in resolution is decisive for detecting the gravitational signature of massive black holes in galaxy nuclei. The discovery of small (r ~ 100-200 pc) rotating gaseous and stellar disks in the centers of many early-type galaxies provides a new and efficient means for measuring the central potentials of galaxies. Concomitantly, VLBI observations of water masers in the nuclei of NGC 4258 and NGC 1068 revealed exquisite Keplerian rotation curves around massive black holes at radii as small as 0.1 pc. Recent terrestrial K-band measurements of the proper motions of stars in the cluster at the center of the Galaxy provide irrefutable evidence for a black hole with a mass of 2.7E+6 M_sun. At the time of this symposium, the presence of central massive black holes has been established in 12 galaxies. The evidence suggests that there are massive black holes in the centers of all AGNs and in most, if not all, nucleated galaxies. The present data show at best a weak correlation between black hole mass and bulge luminosity.