Star Cluster Survival in Star Cluster Complexes under Extreme Residual Gas Expulsion

After the stars of a new, embedded star cluster have formed, they blow the remaining gas out of the cluster. Especially winds of massive stars and definitely the onset of the first supernovae can remove the residual gas from a cluster. This leads to a very violent mass loss and leaves the cluster out of dynamical equilibrium. Standard models predict that within the cluster volume the star formation efficiency (SFE) has to be about 33% for sudden (within 1 crossing time of the cluster) gas expulsion to retain some of the stars in a bound cluster. If the efficiency is lower, the stars of the cluster mostly disperse. Recent observations reveal that in strong starbursts star clusters do not form in isolation, but in complexes containing dozens and up to several hundred star clusters, i.e., in superclusters. By carrying out numerical experiments for such objects placed at distances >=10 kpc from the center of the galaxy, we demonstrate that under these conditions (i.e., the deeper potential of the star cluster complex and the merging process of the star clusters within these superclusters) the SFEs can be as low as 20% and still leave a gravitationally bound stellar population. Such an object resembles the outer Milky Way globular clusters and the faint fuzzy star clusters recently discovered in NGC 1023.