Stellar Velocity Dispersions for Four Low-Concentration Globular Clusters

Mean velocities and velocity dispersions are presented and discussed for four clusters of low central concentration: NGC 5466, NGC 6121 (M4), NGC 6656 (M22), and NGC 6838 (M71). They are derived from measurements with a typical accuracy of + or - 1 km/s made for 12-19 members of each cluster. The measured dispersions range from 1.7 + or - 0.6 km/s for NGC 5466 to 8.5 + or - 1.9 km/s for M22. To within the uncertainties of 20-40 percent, each agrees with the theoretical central-velocity dispersion tabulated by Peterson and King (1975) on the assumption of a solar mass-to-light ratio. This result, in conjunction with those of several other recent investigations of globular clusters, shows that their mass-to-light ratios fall in the range M/L = 0.5-4, whatever their total mass or concentration. Yet in dwarf spheroidal galaxies, mass-to-light ratios may be an order of magnitude larger. If so, the dark matter which dominates those systems is almost entirely absent from globular clusters in the vicinity of the sun.