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.