Globular Clusters and the Potential Well of the Galaxy

Abstract

The globular cluster system affords a unique opportunity to investigate the gravitational field of our Galaxy. Except very close to the Galactic plane the known sample of clusters is almost certainly complete. Both distances and velocities for clusters can be obtained with relative ease to an accuracy which is quite sufficient for the study of Galactic structure, at least outside the central few kiloparsecs. In addition clusters are seen at large radii where they provide information about the potential field that no other test particle population can at present supply. The fundamental limitation of the cluster system in this context is, of course, the fact that less than 100 clusters are known in regions of the Galaxy where we can hope to use them to study Galactic structure. This “small” sample will never be significantly increased and so we are forced to deal with the statistics of small numbers. Further statistical uncertainty is introduced into dynamical analysis of cluster data by the fact that only one of the three components of the space velocity is, in general, observable. Such uncertainties make it impossible to use the clusters to answer detailed questions about the mass distribution of the Galaxy, but their kinematics can give useful information about the large-scale distribution and pose quite severe constraints on any proposed mass model.