The prospect of finding massive BHs in globular clusters motivated much early effort to investigate the distribution of stars resulting from the adiabatic growth of a BH in a preexisting stellar system. The central density deviates strongly from that of an isothermal core and instead follows a cuspy profile (r) r-3/2 (Young 1980) or steeper if two-body relaxation (Peebles 1972; Bahcall & Wolf 1976) or different initial density profiles (Quinlan, Hernquist, & Sigurdsson 1995) are taken into account. The discovery that the centers of some giant elliptical galaxies obey this prediction generated much enthusiasm for the existence of SMBHs. In the well-known case of M87 (Young et al. 1978), Lauer et al. (1992) have since shown that the central cusp persists to the limit of the resolution of the HST (0".1).
However, as emphasized by Kormendy (1993; see also KR), photometric signatures alone do not uniquely predict the presence of a SMBH. The cores of most galaxies are now known to be nonisothermal. And moreover, contrary to naïve expectations, galaxy cores with high central surface brightnesses and small core radii, far from being the ones most likely to host SMBHs, are in fact least expected to do so. This apparently contradictory statement can be most easily understood by considering the so-called fundamental-plane relations for the spheroidal component of galaxies (Faber et al. 1987; Bender, Burstein & Faber 1992). More luminous, more massive galaxies tend to have more massive central BHs (Section 7), but they also have larger, more diffuse cores. Indeed, high-resolution photometric studies of early-type galaxies (Nieto et al. 1991; Crane et al. 1993; Jaffe et al. 1994; Lauer et al. 1995) find that the central surface brightness profiles either continue to rise toward the center as I(r) r-, with 0.5-1.0 (the "power-law" galaxies) or they flatten at some characteristic radius to a shallower slope of 0.0-0.3 (the "core" galaxies). The power-law galaxies are invariably lower luminosity, lower mass systems compared to those with distinct cores.
In summary, photometric signatures alone cannot be used as reliable indicators for the presence of SMBHs. Instead, we must turn to the more arduous task of obtaining kinematic measurements.