Several billion years after the Big Bang, the Universe went through a
``quasar era'' when high-energy active galactic nuclei (AGNs) were more than
10,000 times as numerous as they are now. Quasars must then have been standard
equipment in most large galaxies. Since that time, AGNs have been dying out.
Now quasars are exceedingly rare, and even medium-luminosity AGNs such as
Seyfert galaxies are uncommon. The only activity that still occurs in many
galaxies is weak. A paradigm for what powers this activity is well established
through the observations and theoretical arguments that are outlined in the
previous article. AGN engines are believed to be supermassive black holes
(BHs) that accrete gas and stars and so transform gravitational potential
energy into radiation. Expected BH masses are MBH ~
106 - 109.5
M
. A wide array
of phenomena can be understood within this
picture. But the subject has had an outstanding problem: there was no dynamical
evidence that BHs exist. The search for BHs has therefore become one of the
hottest topics in extragalactic astronomy.
Since most quasars have switched off, dim or dead engines - starving black holes - should be hiding in many nearby galaxies. This means that the BH search need not be confined to the active galaxies that motivated it. In fact, definitive conclusions are much more likely if we observe objects in which we do not, as Alan Dressler has said, ``have a searchlight in our eyes.'' Also, it was necessary to start with the nearest galaxies, because only then could we see close enough to the center so that the BH dominates the dynamics. Since AGNs are rare, nearby galaxies are not particularly active. For these reasons, it is no surprise that the search first succeeded in nearby, inactive galaxies.
This article discusses stellar dynamical evidence for BHs in inactive and weakly active galaxies. Stellar motions are a particularly reliable way to measure masses, because stars cannot be pushed around by nongravitational forces. The price is extra complication in the analysis: the dynamics are collisionless, so random velocities can be different in different directions. This is impossible in a collisional gas. As we shall see, much effort has gone into making sure that unrecognized velocity anisotropy does not lead to systematic errors in mass measurements.
Dynamical evidence for central dark objects has been published for 17 galaxies. With the Hubble Space Telescope (HST) pursuing the search, the number of detections is growing rapidly. Already we can ask demographic questions. Two main results have emerged. First, the numbers and masses of central dark objects are broadly consistent with predictions based on quasar energetics. Second, the central dark mass correlates with the mass of the elliptical-galaxy-like ``bulge'' component of galaxies. What is less secure is the conclusion that the central dark objects must be BHs and not (for example) dense clusters of brown dwarf stars or stellar remnants. Rigorous arguments against such alternatives are available for only two galaxies. Nevertheless, these two objects and the evidence for dark masses at the centers of almost all galaxies that have been observed are taken as strong evidence that the AGN paradigm is essentially correct.