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 (R_gc 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 V_rot ~ 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.