The Massive M31 Cluster G1: Detailed Chemical Abundances from Integrated Light Spectroscopy

TL;DR

This study provides a detailed chemical abundance analysis of the massive M31 cluster G1, revealing its metallicity, alpha-enhancement, and unique Na and Al signatures, suggesting a complex formation history linked to galaxy and cluster evolution.

Contribution

It offers the first integrated light chemical abundance analysis of G1, highlighting its similarities to other M31 GCs and extragalactic nuclear star clusters, and discusses implications for cluster formation.

Findings

G1 is moderately metal-poor with [Fe/H]=-0.98+/-0.05

G1 shows alpha-enhancement indicating a massive galaxy origin

G1 exhibits Na and Al enhancement, linking it to GC formation processes

Abstract

G1, also known as Mayall II, is one of the most massive star clusters in M31. Its mass, ellipticity, and location in the outer halo make it a compelling candidate for a former nuclear star cluster. This paper presents an integrated light abundance analysis of G1, based on a moderately high-resolution (R=15,000) spectrum obtained with the High Resolution Spectrograph on the Hobby-Eberly Telescope in 2007 and 2008. To independently determine the metallicity, a moderate resolution (R~4,000) spectrum of the calcium-II triplet lines in the near-infrared was also obtained with the Astrophysical Research Consortium's 3.5-m telescope at Apache Point Observatory. From the high-resolution spectrum, G1 is found to be a moderately metal-poor cluster, with [Fe/H]=-0.98+/-0.05. G1 also shows signs of alpha-enhancement (based on Mg, Ca, and Ti) and lacks the s-process enhancements seen in dwarf galaxies (based on comparisons of Y, Ba, and Eu), indicating that it originated in a fairly massive galaxy. Intriguingly, G1 also exhibits signs of Na and Al enhancement, a unique signature of GCs -- this suggests that G1's formation is intimately connected with GC formation. G1's high [Na/Fe] also extends previous trends with cluster velocity dispersion to an even higher mass regime, implying that higher mass clusters are more able to retain Na-enhanced ejecta. The effects of intracluster abundance spreads are discussed in a subsequent paper. Ultimately, G1's chemical properties are found to resemble other M31 GCs, though it also shares some similarities with extragalactic nuclear star clusters.