Evidence for Two Distinct Stellar Initial Mass Functions : Revisiting the Effects of Cluster Dynamical Evolution

Abstract

We measure the velocity dispersions of six, galactic globular clusters using spatially integrated spectra, to test for the effects of internal dynamical evolution in the stellar mass-to-light ratios, Upsilon_*, of star clusters. In particular, we revisit whether the low values of Upsilon_* we found in our previous study, from which we concluded that there are at least two population of stellar clusters with distinct stellar initial mass functions, are artificially depressed by relaxation driven mass loss. The combination of our previous sample of five old clusters and these six now provides an order of magnitude range in cluster mass with which to explore this issue. We find no relationship between cluster mass, or relaxation time, and Upsilon_*. Because relaxation is mass dependent, we conclude that the values of Upsilon_* for these clusters are not strongly affected by dynamical effects, and so confirm the presence of the population of clusters with low Upsilon_*.