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

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.

11 pages, 7 figures: accepted for publication in ApJ

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• Controversial Age Spreads from the Main Sequence Turn-Off and Red Clump in Intermediate-Age Clusters in the LMC
• Optical and infrared emission of H II complexes as a clue to the PAH life cycle
• On the progenitor of the Type Ic SN 2013dk in the Antennae Galaxies
• The Compact Star-Forming Complex at the Heart of NGC 253
• Should I Stay or Should I Go: Stellar Wind Retention and Expulsion in Massive Star Clusters
• GalevNB: a conversion from N-BODY simulations to observations
• Young Massive Clusters: Their Population Properties, Formation and Evolution, and Their Relation to the Ancient Globular Clusters
• Resolved photometry of Young Massive Clusters in the starburst galaxy NGC 4214