The Global Kinematics of the Globular Cluster M15

Abstract

We present velocities for 230 stars in the outer parts of the globular cluster M15 measured with the Hydra multi-object spectrograph on the WIYN telescope. A new Bayesian technique is used for analyzing the data. The mean velocity of the cluster is -106.9+/- 0.3 km/s. Rotation with an amplitude of 1.5+/- 0.4 km/s and a position angle of 125+/-19 degrees is observed and the model including rotation is marginally favored over one without rotation. The velocity dispersion decreases from the center out to about 7' and then appears to increase slightly. This behavior is strikingly different from the continued decline of velocity dispersion with increasing radius that is expected in an isolated cluster. We interpret this as an indication of heating of the outer part of M15 by the Galactic tidal field.