The SMBH hunting game is rapidly becoming a rather mature subject. I think we have progressed from the era of "the thrill of discovery" to a point where we are on the verge of using SMBHs as astrophysical tools. In this spirit, let me remark on a few of the ramifications of the existing observations and point out some of the more urgent directions that should be pursued.
A. The
M
-Mbul relation.
The apparent correlation between the mass of the central BH and the
mass of the bulge, if borne out by future scrutiny, has significant
implications (see below). From an observational point of view, the highest
immediate priority is to populate the
M-Mbul
diagram with objects spanning a wide range in luminosity, with the eventual
aim of deriving a mass function for SMBHs. The samples should
be chosen with the following questions in mind. (1) Is the apparent trend a
true correlation or does it instead trace an upper envelope? (2) If the
relation is real, is it linear? (3) What is the magnitude of the intrinsic
scatter? And (4) is there a minimum bulge luminosity (mass) below which
SMBHs do not exist?
In the near future, the most efficient way to obtain mass measurements for relatively large numbers of galaxies is to exploit the capabilities of STIS on HST. Several large programs are in progress. Although VLBI spectroscopy of H2O masers delivers much higher angular resolution, this technique is limited by the availability of suitably bright sources. Conditions which promote H2O megamaser emission evidently are realized in only a tiny fraction of galaxies (Braatz, Wilson, & Henkel 1996).
B. The formation of SMBHs.
The M-Mbul relation offers some clues to
the formation mechanism of SMBHs. How does a galaxy know how to extract a
constant, or at least a limiting, fraction of its bulge mass into a SMBH?
An attractive possibility is by the normal dynamical evolution of the
galaxy core itself. The spheroidal component of nearby galaxies can attain
very high central stellar densities - up to 105
M
pc-3
(Faber et al. 1997)
- and some with distinct nuclei have even higher
concentrations still
(Lauer et al. 1995).
Although most galaxy cores are
unlikely to have experienced dynamical collapse
(Kormendy 1988c),
the innermost regions have much shorter relaxation times,
especially when considering a realistic stellar mass spectrum because
the segregation of the
most massive stars toward the center greatly accelerates the dynamical
evolution of the system. Lee
(1995,
and these proceedings) shows that, under
conditions typical of galactic nuclei, core collapse and merging of
stellar-size BHs can easily form a seed BH of moderate mass.
Alternatively, the seed object may form via
the catastrophic collapse of a relativistic cluster of compact remnants
(Quinlan & Shapiro 1990).
In either case, subsequent accretion of gas and
stars will augment the central mass, and, over a Hubble time, may
produce the
distribution of masses observed. It is far too premature to tell whether
SMBHs form through the secular evolution of galaxies, as suggested here,
through processes associated with the initial formation of galaxies
(e.g.,
Rees 1984,
1998;
Silk & Rees 1998),
or both, depending on the galaxy
type (elliptical vs. spiral galaxies). But the stage is set for a serious
discussion. More sophisticated modeling of the growth of SMBHs that take
into account a wider range of initial conditions in galactic nuclei
(e.g., relaxing the adiabatic assumption or adopting more realistic
density profiles for the
stellar distribution) may eventually yield testable predictions (see, e.g.,
Stiavelli 1998).
C. Influence of SMBHs on galactic structure.
Norman, May, & Van Albada
(1985)
showed through N-body simulations that a massive singularity in
the center of a triaxial galaxy destroys the box-like stellar orbits and
hence can erase the nonaxisymmetry, at least on small scales. This has
several important consequences. First, it implies that the presence of a
SMBH can influence the global structure and dynamics of galaxies.
Second, the secular evolution of the axisymmetry of the central potential
points to a natural mechanism for galaxies to self-regulate the transfer of
angular momentum of the gas from large to small scales. This
negative-feedback
process may limit the growth of the central BH and the
accretion rate onto it, and hence may serve as a promising framework for
understanding the physical evolution of AGNs.
Merritt & Quinlan (1998)
find that the timescale for effecting the transition from triaxiality
to axisymmetry depends strongly on the fractional mass of the BH;
the evolution occurs rapidly when
M
/ Mbul
2.5%,
remarkably close to the observational upper limit
(Fig. 8 a).
D. The origin of central cores. It is not understood why giant ellipticals have such shallow central light profiles. "Cores" do not develop naturally in popular scenarios of structure formation, and even if they form, they are difficult to maintain against the subsequent acquisition of the dense, central regions of satellite galaxies that get accreted (Faber et al. 1997). Moreover, the very presence of a SMBH, whether it grew adiabatically in a preexisting stellar system or the galaxy formed by violent relaxation around it (Stiavelli 1998), ought to imprint a more sharply cusped light profile (see Section 2) than is observed. An intriguing possibility is that cores were created as a result of mergers, where one or more of the galaxies contains a BH. As the single (Nakano & Makino 1998) or binary (Makino 1997; Quinlan & Hernquist 1997) BH sinks toward the center of the remnant due to dynamical friction, it heats the stars, thereby producing a "fluffy" core. If this interpretation is correct, it would provide a simple, powerful tool to diagnose the formation history of galaxies.
E. Why are the black holes so black?
It has been somewhat puzzling how the BHs can remain so
dormant. No doubt the dwindled gas supply in the present epoch, especially
in ellipticals, is largely responsible for the inactivity. Yet the accretion
rate cannot be zero; even in the absence of inflow from the general
interstellar medium, some gas is shed through normal mass loss from the
innermost evolved stars, and occasionally such stars get tidally disrupted
(see below). If
SMBHs are indeed present, the radiative efficiency of the accretion flow
must be orders of magnitude lower than that of "standard" optically thick,
geometrically thin disks. Such a situation may be realized in accretion
flows where advection becomes important when the accretion rate is highly
sub-Eddington
(Narayan & Yi 1995;
Abramowicz et al. 1995;
Nakamura et al. 1996;
Chakrabarti 1996).
Sgr A* at the Galactic Center has a bolometric
luminosity of only ~ 1037 ergs s-1, or
Lbol / LEdd
3 × 10-8
(Narayan, Yi, & Mahadevan
1995);
in the case of the LINER nucleus of M81
(Ho, Filippenko, & Sargent
1996),
Lbol
1041 ergs s-1 and
Lbol / LEdd
10-4. The
spectral energy distributions emitted by both objects differ
dramatically from those of
luminous AGNs and can be approximately matched by advective-disk models.
F. Tidal disruption of stars.
The prevalence of SMBHs suggested by the existing evidence predicts a
relatively high incidence of tidal disruptions of stars as they scatter
into nearly radial orbits whose pericenters pass within the tidal radius
of the BH
(Rees 1998,
and references therein). For a typical stellar
density of 105 stars pc-3,
M =
106-108
M,
and 
= 100-300 km
s-1, a solar-type star will be disrupted once
every 102-104 years. (BHs more massive than
108
M
will swallow the star whole.) Roughly half the debris becomes unbound and
half gets captured into an accretion disk which undergoes a bright flare
(~ 1010
L) lasting a few
months to a year. The spectrum is
expected to be mainly thermal and to peak in the extreme-UV and soft
X-rays. The contribution to the near-UV and optical bands is uncertain;
it depends on assumptions concerning the geometry of the accretion disk
(thick or thin) and
on whether an optically thick envelope can form. For plausible parameters,
Ulmer (1998)
estimates that a 107
M BH will produce a
flare
with an absolute magnitude of about -20 in U and -18.5 in V.
The realization that SMBHs may be even more common than previously thought
provides fresh motivation to search for such stellar flares; some
observational strategies are mentioned by
Rees (1998).
Here, I wish to stress that
quantifying the rate of stellar disruptions can be used as a tool to study
the demography of SMBHs out to relatively large distances and hence
should be regarded as complementary to the kinematic searches.
Acknowledgements
I am grateful to S. K. Chakrabarti for the invitation to participate in this workshop and his help in arranging a pleasurable visit to India. I thank G. A. Bower, S. Collier, R. Genzel, L. J. Greenhill, J. Kormendy, R. Maiolino, D. Maoz, E. Maoz, and K. Nandra for contributing to, or for providing comments that have improved the presentation of, the material in this paper. This work was supported by a postdoctoral fellowship from the Harvard-Smithsonian Center for Astrophysics and by NASA grants from the Space Telescope Science Institute (operated by AURA, Inc., under NASA contract NAS 5-26555).