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

The robustness of stellar-dynamical black hole (BH) mass measurements is illustrated using six galaxies that have results from independent research groups. Derived BH masses have remained constant to a factor of ∼ 2 as spatial resolution has improved by a factor of 2 – 330, as velocity distributions have been measured in increasing detail, 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. Another successful test is the agreement between a preliminary stellar-dynamical BH mass for NGC 4258 and the accurate mass provided by the maser disk. Constraints on BH alternatives are also improving. In M 31, 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 have a radius r < ∼ 0. 06. M 31 becomes the third galaxy in which dark clusters of brown dwarf stars or stellar remnants can be excluded. In our Galaxy, spectacular proper motion observations of almost-complete stellar orbits show that the central dark object has radius r < ∼ 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 (AGNs) is correspondingly high. Compared to the radius of the BH sphere of influence, BHs are being discovered at similar spatial resolution with HST as in ground-based work. The reason is that HST is used to observe more distant galaxies. Typical BHs are detectable in the Virgo cluster, and the most massive ones are detectable 3 – 6 times farther away. Large, unbiased samples are accessible. As a result, HST has revolutionized the study of BH demographics. 1.