Thus far we have rigorously shown only that many galaxies contain central MDOs, not that the dark masses must be in the form of SMBHs. Direct proof of the existence of SMBHs would require the detection of relativistic motions emanating from the vicinity of the Schwarzschild radius, RS = 2GM / c2 10-5(M / 108 M) pc. Even for our neighbor M31, RS subtends 3 × 10-6 arcseconds, and the Galactic Center only a factor of 2 larger. We are clearly still far from being able to achieve the requisite angular resolution and in the meantime must rely on indirect arguments.
One approach seeks to identify some observational feature that might be taken as a fingerprint of the event horizon or of physical processes uniquely associated with the environment of a BH. One such "signature" might be the broad Fe K line discussed in Section 5; another is the high-energy power-law tail observed in some AGNs and Galactic BH candidates (Titarchuk & Zannias 1998). And yet a third possibility is the advection of matter into the event horizon (Menou, Quataert, & Narayan 1999).
A different strategy appeals to the dynamical stability of the probable alternative sources of the dark mass (Goodman & Lee 1989; Richstone, Bower, & Dressler 1990; van der Marel et al. 1997; Maoz 1998). The absence of strong radial gradients in the stellar population, as measured by variations in color or spectral indices, implies that the large increase in M / L toward the center cannot be attributed to a cluster of ordinary stars. On the other hand, the underluminous mass could, in principle, be a cluster of stellar remnants (white dwarfs, neutron stars, and stellar-size BHs) or perhaps even substellar objects (planets and brown dwarfs). To rule out these possibilities, however exotic they might seem, one must show that the clusters cannot have survived over the age of the galaxy, and hence finding them would be highly improbable.
As most recently discussed by Maoz (1998), the two main processes that determine the lifetime of a star cluster are evaporation, whereby stars escape the cluster as a result of multiple weak gravitational scatterings, and physical collisions among the stars themselves. Exactly which dominates depends on the composition and size of the cluster, and its maximum possible lifetime can be computed for any given mass and density. Maoz (1998) shows that in two galaxies, namely the Milky Way and NGC 4258, the density of the dark mass is so high ( 1012 M pc-3) that it cannot possibly be in the form of a stable cluster of stellar or substellar remnants: their maximum ages [~ (1-few) × 108 yr] are much less than the ages of the galaxies. The only remaining constituents allowed appear to be subsolar-mass BHs and elementary particles. This constitutes very strong evidence that the MDOs - at least in two cases - are most likely SMBHs. In the following discussion, I will adopt the simplifying viewpoint that all MDOs are SMBHs, bearing in mind that at the current resolution limit we cannot yet disprove the dark-cluster hypothesis for the majority of the objects.