Multiple Populations in Integrated Light Spectroscopy of Intermediate Age Clusters

TL;DR

This study investigates the presence of multiple stellar populations in intermediate-age clusters using integrated light spectroscopy, finding evidence of such populations in a massive cluster, which supports the idea that young massive clusters are similar to ancient globular clusters.

Contribution

It demonstrates the feasibility of detecting multiple populations in integrated light spectra of intermediate-age clusters, especially in very massive clusters, expanding the methods for studying stellar populations.

Findings

No Na spreads in NGC 1978 consistent with resolved star studies

Clear evidence of multiple populations in G114

Supports similarity between young massive and ancient globular clusters

Abstract

The presence of star-to-star light-element abundance variations (a.k.a. multiple populations, MPs) appears to be ubiquitous within old and massive clusters in the Milky Way and all studied nearby galaxies. Most previous studies have focussed on resolved images or spectroscopy of individual stars, although there has been significant effort in the past few years to look for multiple population signatures in integrated light spectroscopy. If proven feasible, integrated light studies offer a potential way to vastly open parameter space, as clusters out to tens of Mpc can be studied. We use the NaD lines in the integrated spectra of two clusters with similar ages ($2-3$ Gyr) but very different masses, NGC 1978 ($\sim3\times10^5$ Msun) in the LMC and G114 ($1.7\times10^7$ Msun) in NGC 1316. For NGC 1978, our findings agree with resolved studies of individual stars which did not find evidence for Na spreads. However, for G114, we find clear evidence for the presence of multiple populations. The fact that the same anomalous abundance patterns are found in both the intermediate age and ancient GCs lends further support to the notion that young massive clusters are effectively the same as the ancient globular clusters, only separated in age.