New Constraints on Mass-Dependent Disruption of Star Clusters in M51

Abstract: We use UBVI,Ha images of the Whirlpool galaxy, M51, taken with the ACS and WFPC2 cameras on the Hubble Space Telescope (HST) to select star clusters, and to estimate their masses and ages by comparing their observed colors with predictions from population synthesis models. We construct the mass function of intermediate age (1-4x10^8 yr) clusters, and find that it is well described by a power law, psi(M) propto M^beta, with beta=-2.1 +/- 0.2, for clusters more massive than approximately 6x10^3 Msun. This extends the mass function of intermediate age clusters in M51 to masses lower by nearly a factor of five over previous determinations. The mass function does not show evidence for curvature at either the high or low mass end. This shape indicates that there is no evidence for the earlier disruption of lower mass clusters compared with their higher mass counterparts (i.e., no mass-dependent disruption) over the observed range of masses and ages, or for a physical upper mass limit Mc with which clusters in M51 can form. These conclusions differ from previous suggestions based on poorer-quality HST observations. We discuss their implications for the formation and disruption of the clusters. Ages of clusters in two "feathers," stellar features extending from the outer portion of a spiral arm, show that the feather with a larger pitch angle formed earlier, and over a longer period, than the other.