Signatures of multiple stellar populations in unresolved extragalactic globular/ young massive star clusters

TL;DR

This study identifies observational signatures of multiple stellar populations in unresolved extragalactic star clusters, providing evidence of complex formation histories similar to those seen in Galactic globular clusters.

Contribution

The paper demonstrates that unresolved spectra of young extragalactic clusters can reveal multiple stellar populations through combined emission and absorption lines, a novel observational approach.

Findings

Detected two stellar populations in a cluster in the Antennae galaxies.

Populations are physically separated by about 59 pc.

Clusters may coalesce to form globular clusters with multiple populations.

Abstract

We present an investigation of potential signatures of the formation of multiple stellar populations in recently formed extragalactic star clusters. All of the Galactic globular clusters for which good samples of individual stellar abundances are available show evidence for multiple populations. This appears to require that multiple episodes of star formation and light element enrichment are the norm in the history of a globular cluster. We show that there are detectable observational signatures of multiple formation events in the unresolved spectra of massive, young extragalactic star clusters. We present the results of a pilot program to search for one of the cleanest signatures that we identify - the combined presence of emission lines from a very recently formed population and absorption lines from a somewhat older population. A possible example of such a system is identified in the Antennae galaxies. This source's spectrum shows evidence of two stellar populations with ages of 8 Myr and 80 Myr. Further investigation shows that these populations are in fact physically separated, but only by a projected distance of 59 pc. We show that the clusters are consistent with being bound and discuss the possibility that their coalescence could result in a single globular cluster hosting multiple stellar populations. While not the prototypical system proposed by most theories of the formation of multiple populations in clusters, the detection of this system in a small sample is both encouraging and interesting. Our investigation suggests that expanded surveys of massive young star clusters should detect more clusters with such signatures.