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BLACK HOLES IN GALACTIC NUCLEI 1
The dynamical evidence
ROELAND P. VAN DER MAREL 2
Institute for Advanced Study
Olden Lane, Princeton, NJ 08540, USA
Abstract. The dynamical evidence for black holes (BHs) in galactic nuclei
is reviewed, with emphasis on recent improvements in spatial resolution,
methods for analyzing galaxy spectra and dynamical modeling. M31, M32
and M87 are discussed in some detail.
1. Introduction
Most models of the energy production in quasars and active galactic nuclei
(AGN) invoke the presence of massive black holes, with MBH ú 10 6 \Gamma 10 9
M fi (e.g., Rees 1984). The observed number of quasars at high redshifts
implies that many of the normal galaxies today must have gone through
an active phase in the past. BHs should thus be common in the nuclei of
quiescent galaxies as well (Lynden­Bell 1969; Chokshi & Turner 1992).
The gravitational attraction of a BH in a galactic nucleus influences the
distribution and dynamics of the surrounding stars and gas. There will be
a central power­law cusp in the stellar surface brightness profile (Bahcall &
Wolf 1976; Young 1980; Cipollina & Bertin 1994; Quinlan et al. 1994). The
stellar and gas velocities (mean or RMS) will be Keplerian, i.e., v / r \Gamma1=2 .
Spatial resolution is the main difficulty in detecting these effects. The
influence of a nuclear BH extends approximately to a radius r = GMBH =oe 2 ,
where oe is the galaxy's virial velocity. In practice this radius is often Ÿ 1 00 ,
of the same order as the seeing FWHM for ground­based observations.
2. Overview
Hubble Space Telescope (HST) photometry has shown that most elliptical
galaxies have central surface brightness cusps (e.g., Crane et al. 1993; Fer­
1 To appear in: Highlights of Astronomy, Vol. 10, Proc. of the XXIInd General Assem­
bly of the IAU, The Hague, August 1994, ed. J. Bergeron, Kluwer Academic Publishers.
2 Hubble Fellow.

2 ROELAND P. VAN DER MAREL
rarese et al. 1994; Kormendy et al. 1994), consistent with the predictions of
models with BHs. However, other physical processes can also lead to high
central mass densities in galaxies (e.g., Kormendy 1993). Kinematical data
are thus required to firmly establish the presence of BHs.
The gas kinematics in the narrow­ and broad­line regions of AGN has
been much studied (e.g., Whittle 1994). Gas motions ? 1000 km/s can be
observed. However, it remains ill­established whether the motions are grav­
itational, due to in­ or outflow, or chaotic (e.g., Osterbrock 1991), mainly
because the emitting regions are not spatially resolved from the ground.
The motions can thus not be used directly to trace the gravitational poten­
tial. This will improve with high spatial resolution spectroscopy from the
refurbished HST, like that obtained already for M87 (Section 3).
Ground­based stellar kinematical studies have produced tentative evi­
dence for the presence of BHs in M31, M32, M87, NGC 3115 and NGC
4594 (e.g., Kormendy 1993), but models without a BH have not been con­
vincingly ruled out. The unknown stellar velocity dispersion anisotropy
introduces the main uncertainty. Models with an excess of stars on radial
orbits predict a central peak in the RMS stellar velocity, very similar to
what is predicted in isotropic models with a BH (Binney & Mamon 1982).
The dynamical modeling of kinematical data has recently improved sig­
nificantly. Traditionally, spherical or isotropic models were used (in fact,
our understanding of these models is still improving: Tremaine et al. 1994;
van der Marel 1994d). Now, more sophisticated flattened models are avail­
able. Those with a distribution function f(E; L z
) have been particularly
well studied (e.g., Hunter & Qian 1993; Dehnen & Gerhard 1994; van der
Marel et al. 1994b; Qian et al. 1994), but more general models can also be
constructed (Dehnen & Gerhard 1993).
Methods for analyzing absorption­line spectra have traditionally as­
sumed the stellar line­of­sight velocity profiles (VPs) to be Gaussian. Re­
cently, new techniques have been developed that allow the actual VP shapes
to be measured (e.g., Rix & White 1992; van der Marel & Franx 1993). Van
der Marel et al. 1994a,b,c observed the nearby galaxies with suspected BHs,
and measured and modeled their VPs. These data contain new information
on the dynamical structure of these galaxies, and allow more stringent con­
straints to be placed on the presence of BHs.
Stellar kinematical HST data, soon to be expected, should provide much
new information. If a BH is present in a galaxy, the central VP will have
broader wings than a Gaussian. Gaussian VP fits will strongly underes­
timate the true velocity dispersion (van der Marel 1994d). Modeling and
analysis of VP shapes is thus essential. Flattened models will generally be
required, since many galaxies show nuclear stellar disks when viewed with
HST (van den Bosch et al. 1994).

BLACK HOLES IN QUIESCENT GALAXIES 3
3. Some individual cases
For M31, M32 and M87 much progress has been made recently:
M31: Kormendy (1988a) and Dressler & Richstone (1988) interpreted
kinematical data for M31 by invoking the presence of a 10 7 \Gamma 10 8 M fi BH.
HST photometry by Lauer et al. (1993) showed that M31 has a double
nucleus. Bacon et al. (1994) mapped the stellar kinematics of M31 with the
two­dimensional TIGER spectrograph at the CFHT. The faintest of the two
nuclei is at the center of the bulge isophotes, and is the kinematical center
of the (remarkably regular) velocity field. The velocity dispersion peak is
offset from both nuclei. Several explanations for the observed nuclear asym­
metries have been discussed, but no consensus has yet been reached. The
central regions of M31 might not be in dynamical equilibrium, and the case
for a BH (or even two BHs) thus remains ambiguous.
M32: Models and observations by Tonry (1987) and Dressler & Richstone
(1988) suggested the presence of a BH in M32. Lauer et al. (1992b) pre­
sented HST photometry of M32 showing an I / r \Gamma0:5 surface brightness
cusp, and argued for a BH on the basis of collision­ and relaxation­time
arguments. Van der Marel et al. (1994b) and Qian et al. (1994) constructed
flattened dynamical models with f = f(E; L z ) to interpret the kinematical
data along five different slit position angles presented by van der Marel et
al. (1994a). These models require the presence of a 1:8 \Theta 10 6 M fi BH, and fit
all the data, including the VP deviations from Gaussians, with remarkable
accuracy. This does not rule out alternative models, but does put the BH
case on much stronger footing. HST observations should provide impor­
tant new information. The BH model predicts a central velocity dispersion
of 129 km/s with the 0:1 00 square aperture of the HST/FOS. The central
velocity dispersion measured from the ground is only 85 km/s.
M87: HST photometry by Lauer et al. (1992a) showed that M87 has an
I / r \Gamma0:26 surface brightness cusp. This confirmed photometry by Young
et al. (1978), who invoked a 2:6 \Theta 10 9 M fi BH as explanation. Sargent et
al. (1978) invoked a BH to fit the stellar kinematics. In the 1980's this
evidence was demonstrated to be ambiguous. Van der Marel (1994c) showed
that the best ground­based stellar kinematical data can still be fit equally
well with models with a 3 \Theta 10 9 M fi BH, as with models without a BH.
He also detected very rapid ionized gas motions near the nucleus of M87,
suggested the motions to be gravitational, and demonstrated that the gas
motions then imply a 3 \Theta 10 9 M fi BH. Beautiful HST observations of the
ionized gas have confirmed this hypothesis: the gas lies in a disk (Ford

4 ROELAND P. VAN DER MAREL
et al. 1994), and rotates with an amplitude of 500 km/s at 0:3 00 (Harms et
al. 1994). This, almost unambiguously, implies the presence of a 2:4\Theta10 9 M fi
BH. Since M87 is a known active galaxy with an optical synchrotron jet, it
does not answer the question whether quiescent galaxies also have BHs.
Support for this work was provided by NASA through a Hubble Fellow­
ship, #HF­1065.01­94A, awarded by the Space Telescope Science Institute
which is operated by AURA, Inc., for NASA under contract NAS5­26555.
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