Annu. Rev. Astron. Astrophys. 1994. 32:
531-590
Copyright © 1994 by . All rights reserved |

**6.5. Constraints on Dark Objects Outside Halos**

Dynamical constraints on dark objects in the Galactic disk are generally
stronger than the halo limits. In particular,
Bahcall et al (1985)
have argued that the disk
dark matter could not comprise objects larger than 2
*M*_{} or else
they would disrupt the wide binaries observed by
Latham et al (1984).
[This limit can be deduced from Equation (6.2) by identifying
*m*_{c} and *r*_{c} with the total mass and
separation of the binary.] This is an important constraint because, if
correct, it
rules out disk dark matter comprising stellar black holes. However, the
Bahcall et al conclusion has been disputed by
Wasserman & Weinberg (1987)
on the grounds that there is no sharp cut-off in the distribution of
binary separations
above 0.1 pc. The limit is therefore weakened (somewhat arbitrarily) to
10 *M*_{}
in Figure 3.

Dynamical constraints on dark objects in clusters of galaxies are weaker
than the halo limits. For example, one does not get an interesting
constraint by
applying Equation (6.2) to the disruption of cluster galaxies by cluster
black holes because the upper limit on
_{B}
exceeds the cluster density. However, one
does get an interesting constraint from upper limits on the fraction of
galaxies
*f*_{g} with unexplained tidal distortions. Equation (6.2)
can also be applied in this
case, except that the limits are weakened by a factor
*f*^{-1}_{g}, where the parameter
(~ 2) represents the
difference between distortion and disruption.
Van den Bergh (1969)
applied this argument to the Virgo cluster and inferred that black
holes binding the cluster could not be bigger than 10^{9}
*M*_{}. If we
assume that Virgo is typical, we obtain the limit indicated in
Figure 3. We also show the
limit corresponding to the requirement that there be at least one black
hole of mass *M* within the cluster.

The dynamical constraints on intergalactic black holes are even weaker. The
most interesting one comes from the fact that, if there were a population of
huge intergalactic black holes, each galaxy would have a peculiar
velocity due to its gravitational interaction with the nearest one
(Carr 1978).
If the holes
were smoothly distributed and had a number density *n*, one would
expect every
galaxy to have a peculiar velocity of order
*GMn*^{2/3} *t*_{g}. Since the CMB dipole
anisotropy shows that the peculiar velocity of our own Galaxy is only 600
km s^{-1}, one infers a limit
_{B} <
(*M* / 10^{16}
*M*_{})^{-1/2} and this is also shown in
Figure 3. The limit on the bottom
right corresponds to the requirement that
there be at least one object of mass *M* within the current
particle horizon.