The Global Kinematics of the Globular Cluster M92

Abstract

We report the determination of high-accuracy radial velocities for 299 members of the globular cluster M92 using the Hydra multi-object spectrograph on the WIYN telescope. We have concentrated on stars outside of the central region of the cluster, located up to 14'4 from the cluster center. Candidate members were selected for spectroscopy based on a photometric metallicity index determined from 3-band Washington photometry, also obtained with the WIYN telescope. The median error in the velocities is 0.35 km/s. We find the heliocentric radial velocity of the cluster to be -121.2 +/-0.3 km/s. We have used an improved Bayesian analysis to determine the velocity dispersion profile of M92. The most probable profile is a cored power-law with a scale radius of 2', velocity dispersion at 1' of 6.3km/s and outer power-law with slope -0.6. We have also reanalyzed the M15 radial velocities of Drukier et al. (1998) and find that a pure power-law with a 1' velocity dispersion of 8 km/s and slope -0.5, and the combination of a power-law with slope -0.4 and scale of 7.5 km/s inside 9' and a dispersion of 4 km/s outside, are equally likely. In both clusters there is evidence that the samples include escaping stars. We present results from a GRAPE-based N-body simulation of an isolated cluster that demonstrates this effect. We suggest additional tests to determine the relative importance of tidal heating and stellar ejection for establishing the velocity field in globular cluster halos.

55 pages, 15 figures. To appear in AJ

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