The Stellar-Dynamical Search for Supermassive Black Holes in Galactic Nuclei

The robustness of stellar-dynamical black hole (BH) mass measurements is illustrated using 7 galaxies that have results from independent groups. Derived masses have remained constant to a factor of about 2 as spatial resolution has improved by factors of 2 - 330 and as the analysis has improved from spherical, isotropic models to axisymmetric, three-integral models. This gives us confidence that the masses are reliable and that the galaxies do not indulge in a wide variety of perverse orbital structures. Constraints on BH alternatives are also improving. In M31, Hubble Space Telescope (HST) spectroscopy shows that the central massive dark object (MDO) is in a tiny cluster of blue stars embedded in the P2 nucleus of the galaxy. The MDO must be less than about 0.06 arcsec in radius. M31 becomes the third galaxy in which dark clusters of brown dwarf stars or stellar remnants can be excluded. In our Galaxy, observations of almost-complete stellar orbits show that the MDO radius is less than about 0.0006 pc. Among BH alternatives, this excludes even neutrino balls. Therefore, measurements of central dark masses and the conclusion that these are BHs have both stood the test of time. Confidence in the BH paradigm for active galactic nuclei is correspondingly high. Compared to the radius of the BH sphere of influence, BHs are discovered at similar spatial resolution with HST as in ground-based work. The reason is that HST is used to observe more distant galaxies. Large, unbiased samples are accessible. As a result, HST has revolutionized the study of BH demographics.