Survival Rates and Consequences

To first order, the initial cluster luminosity function appears to be universal. This means that the brightest young cluster in a galaxy can be predicted from the total number of young clusters based purely on statistics. This suggests that the physical processes responsible for the formation of clusters are similar in a wide variety of galaxies, from mergers to quiescent spirals. One possibility is that conditions for making young massive clusters are globally present in mergers while only locally present in spirals (i.e., in the spiral arms). However, understanding the destruction of clusters and the accompanying survival rates is more important for understanding cluster demographics than understanding their formation. This is because only about 1 in 1,000 clusters with mass greater than 10 M⊙ will survive to become an old globular cluster. In this paper we briefly review this basic framework and then develop a toy model that allows us to begin to address several fundamental questions. In particular, we demonstrate that young clusters in the Antennae Galaxies have a high “infant mortality” rate, with roughly 90 % of the clusters being destroyed each decade of log(time). We also advocate the use of an objective classification system for clusters, with the three parameters being mass, age, and size.