The kinematics and dynamics of the galactic globular cluster system

Abstract

We formulate a general method for testing kinematical models of the globular cluster system in our Galaxy against observational data. Using a sample of 66 clusters with reliable distance and velocity determinations, we find the range of possible models to be significantly constrained. Our data are consistent with a systemic rotation velocity for the cluster population independent both of position and of cluster metallicity and taking the value 60 ± 26 km s⁻¹ for an assumed galactic rotation velocity of 220 km s⁻¹ on the solar circle. The observed velocity dispersion of the cluster system increases significantly with galactocentric distance and is inconsistent at the 95 per cent confidence level with any isothermal or polytropic distribution function. If the mean eccentricity of cluster orbits is allowed to depend on their scale, a wide range of eccentricity behaviour is consistent with the data. However, all acceptable models imply galactic rotation curves which are either flat or rise continuously with increasing radius. In addition all acceptable models predict a low value for the circular velocity at the solar radius and our most plausible models suggest $$200\enspace\text {km}\enspace \text s^{-1}\lesssim\upsilon_\odot\lesssim225\enspace \text {km}\enspace \text s^{-1}$$. The globular cluster data thus demonstrate the existence of a massive halo in the Milky Way extending at least to the limit of our sample (33 kpc), and they confirm recent downward revisions of the amplitude of the galactic rotation curve.