5.1 Velocity Dispersions for Elliptical Galaxies in Clusters
The velocity dispersion profiles for most elliptical galaxies are flat
or falling. However,
Dressler (1979) and
Carter et al. (1985)
have observed rising rotation curves in
the outer regions of several cD galaxies. This provides evidence that
the cluster
environment can affect the distribution of mass for galaxies in the
centers of clusters,
and more specifically, provides support for the cannibalism or tidal
accretion theory for cD formation.
Dressler (1979)
argues that three components are required to fit his
data; an inner region similar to normal elliptical galaxies (M/L
10),
an intermediate
region that might represent accreted cluster members (M/L
35), and an extended
cluster-filling halo (M/L
500). Although this would agree with several of the
arguments presented in Section 4.1, more
extensive observational data is clearly needed to
make the case based on velocity dispersion gradients alone. Observations
of the globular cluster system
(Mould, Oke, and Nemec
1987)
and the X-ray halo around M87
(Fabricant and
Gorenstein 1983)
also provide evidence that the mass
around central cluster galaxies can grow to prodigious amounts.
On a smaller scale, Dressler
(1984b,
1987)
and Dressler et
al. (1987)
found that the Faber-Jackson and the
Dn-
relations for
both ellipticals and spiral bulges in clusters
are similar to their counterpart relations in the field. There is no
apparent difference in
the Faber-Jackson relation for cD galaxies once light from the outer
envelope is removed from the comparison
(Malumuth and Kirshner
1981).
Even severely
interacting galaxies have a similar Faber-Jackson relation
(Lake and Dressler 1986).
It appears that the
strongly bound stars near the centers of galaxies are not unduly
affected by the environment. However,
Djorgovski, de
Carvalho, and Han (1989)
find tentative evidence that
both the Dn- and
Tully-Fisher relations may be affected by the cluster environment at
the few percent level, and caution that the assumption that these
relations are universal
may lead to systematic errors in recent observations of large-scale motions.
These studies all use measurements of the central velocity dispersion where environmental effects may be minimized. The question of what is happening to the velocity dispersion gradients in the outer regions of normal cluster ellipticals has not yet been addressed.