4.1. Velocities and Kinematics
A new line of investigation which is being made increasingly possible
by the 8- and 10-meter generation of telescopes is the kinematical
analysis of these GCSs by large samples of radial velocities.
Cohen & Ryzhov (1997),
from accurate new
velocity and abundance measurements of more than 200 clusters
in M87, argue that virtually all the clusters (MRC or
MPC) have
traditionally ``old'' ages from their integrated line strengths,
and use the radial velocity dispersion as a function of radius
to trace out the mass profile of the halo.
Kissler-Patig &
Gebhardt (1998),
using Cohen & Ryzhov's data,
demonstrate that the outer part of the GCS (Rgc
20 kpc)
shows substantial net rotation increasing past 200 km s-1,
along the major axis of the halo isophotes (dominated by the cD
envelope). By contrast,
the region inside ~ 20 kpc shows little significant
rotation. Most of the rotation signal seems to arise from the
metal-poor component, while the metal-richer clusters
have a more modest and more nearly uniform Vrot ~ 100
km s-1 at all radii.
This situation is in contrast to what we see in the Milky Way halo.
Kissler-Patig & Gebhardt suggest that this angular momentum pattern
was the result of a single merger between two already-large galaxies.
Such a conclusion may not be the only possible one, since
outward transport of angular momentum will result from a wide
variety of formation scenarios involving either dissipational
collapse or multiple mergers (or both), and even the merger
product of several large galaxies can end up with a large
outer-halo rotation (e.g.,
Weil & Hernquist 1996).
For NGC 4472, which has a total luminosity comparable to M87 but lacks a cD envelope or the ``special'' central position in the Virgo cluster potential well, similar velocity data have begun to accumulate. Sharples et al. (1998) have demonstrated from a new sample of cluster radial velocities that the MRC forms a dynamically ``cooler'' subsystem with distinctly lower velocity dispersion. In analogy with M87, there are hints that the MPC has a distinctly higher net rotation as well, though more data will be needed to confirm this.
The kinematical analysis of GCSs in other galaxies is just getting started. Large samples of velocities are essential to this type of study, and we can look forward in a few years to extremely informative new comparisons with theory.