Dynamical Correlations for Globular Clusters in M31

Abstract

We present internal velocity dispersion measurements for a set of 21 globular clusters in the Andromeda galaxy (M31). We combine them with structural and photometric cluster parameters measured earlier with the Hubble Space Telescope and from the ground to explore correlations of cluster properties and to compare them with the equivalent correlations for the Galactic globular clusters. We find that the M31 globulars follow the same correlations between velocity dispersion and luminosity, central, and average surface brightness, as do their Galactic counterparts. This suggests a common physical origin for these correlations. They may be produced by the same astrophysical conditions and processes operating at the epoch of globular cluster formation in both galaxies. The very existence of these excellent correlations, and their quantitative form as scaling laws, represent challenges and constraints for theories of globular cluster formation. Preliminary estimates of the cluster M/L ratios show correlations with the cluster metallicity, in the sense of more metal-rich clusters having lower M/L, particularly in the near-infrared. At a given metallicity, there is no detectable systematic difference between the M31 globulars and their Galactic counterparts, which suggests a great similarity of their stellar populations. The observed scatter around these trends is comparable to the expected errors, which implies a small intrinsic scatter and thus which leaves little room for possible variations in the cluster age or stellar IMF at a given metallicity.