Dynamics of the Globular Cluster NGC 6656 with WFPC2

We have used the Hubble Space Telescope observations to measure proper motion of the globular cluster NGC 6656 (M22) with respect to the background bulge stars and its internal velocity dispersion profile. Based on the proper motion of the cluster, its space velocity (II,,W) = (184±3,209±14, 132±15)kms^-1 and galactic orbit are also obtained. The central velocity dispersion in radial and tangential components of the internal motion of cluster stars is 16.99kms^-1. We derive the mass-to-light ratio M/L_v～3.3±0.2, which is relatively higher than the previous results.