Evidence for the strong effect of gas removal on the internal dynamics of young stellar clusters

We present detailed luminosity profiles of the young massive clusters M82-F, NGC 1569-A and NGC 1705-1 which show significant departures from equilibrium ( King and Elson, Fall & Freeman) profiles. We compare these profiles with those from N-body simulations of clusters that have undergone the rapid removal of a significant fraction of their mass as a result of gas expulsion. We show that the observations and simulations agree very well with each other, suggesting that these young clusters are undergoing violent relaxation and are also losing a significant fraction of their stellar mass. That these clusters are not in equilibrium can explain the discrepant mass-to-light ratios observed in many young clusters with respect to simple stellar population models without resorting to non- standard initial stellar mass functions as claimed for M82-F and NGC 1705-1. We also discuss the effect of rapid gas removal on the complete disruption of a large fraction of young massive clusters ('infant mortality'). Finally, we note that even bound clusters may lose > 50 per cent of their initial stellar mass as a result of rapid gas loss ('infant weight-loss').