Dynamics of the Globular Cluster System Associated with M49 (NGC 4472): Cluster Orbital Properties and the Distribution of Dark Matter*

Using the Low Resolution Imaging Spectrometer on the Keck I and II telescopes, we have measured radial velocities for 196 globular clusters (GCs) around M49 (NGC 4472), the brightest member of the Virgo Cluster. Combined with published data, they bring the total number of GCs with measured radial velocities in this galaxy to 263. In terms of sample size, spatial coverage, velocity precision, and the availability of metallicity estimates from Washington photometry, this radial velocity database resembles that presented recently for M87 (NGC 4486), Virgo's cD galaxy and its second-ranked member. We extract the projected kinematics of the full sample of GCs and of separate subsamples of 158 metal-poor and 105 metal-rich GCs. In agreement with previous results for the global GC kinematics based on smaller data sets, we find that the GC system as a whole exhibits a slow overall rotation that is due almost entirely to a net rotation of the metal-poor GC subsystem alone. In a spatial average, the metal-rich GCs show essentially no rotation. As a function of galactocentric position, the metal-poor GCs rotate roughly about the photometric minor axis of M49 and at an approximately constant level of ΩR ~ 100-150 km s-1 out to R 2Reff. The metal-rich GC system shows some evidence (at roughly 1 σ significance) for weak rotation (ΩR ~ 50 km s-1) beyond R 0.5Reff, also about the galaxy's minor axis, but in the opposite direction from the metal-poor GCs. Outside of R ~ Reff, the line-of-sight velocity dispersion of the metal-poor GCs exceeds that of their metal-rich counterparts by ~50%. We also note the presence of a well-defined grouping of 10 metal-rich GCs that are located at opposite poles along the galaxy's major axis and that appear to be rotating at nearly 300 km s-1 about the minor axis. This grouping may be the relic of a past merger or accretion event. The dynamics of the GC system is modeled by using published catalogs and number counts to define three-dimensional GC density distributions as input to a Jeans equation analysis. We show that the GC radial velocities alone point unequivocally, and independently of X-ray observations, to the need for a massive dark halo associated with M49 and the Virgo B subcluster around it. We then use a mass model for M49/Virgo B, constructed without reference to any GC data and described in detail in a forthcoming paper, to infer the orbital properties of the M49 globulars. The GC system as a whole is shown to be consistent with an almost perfectly isotropic velocity ellipsoid. It is more difficult to draw any firm conclusions on the orbital (an)isotropy of the two metallicity subsamples, as a result of the large uncertainties in their individual spatial density profiles and the poorly observationally defined kinematics of the metal-rich GCs in particular. After M87, M49 is the second elliptical galaxy for which we have been able to demonstrate velocity isotropy in the GC system overall, when no division based on GC color or metallicity is attempted. Thus, the data for these two galaxies lend support to the general assumption of isotropy when using GC kinematics to study the dark matter distribution in early-type galaxies. We also compare the kinematic properties of the GC system of M49 to those of M87, M31, and the Milky Way, the other galaxies for which samples of 100 or more GC velocities have been accumulated. We argue that, contrary to the traditional view of GCs as nonrotating, or slowly rotating, systems, rotation may in fact be a common by-product of the formation of GC systems. However, the quantitative details of the rotation are still not clear, particularly with regard to the question of possible differences between metal-poor and metal-rich globulars.