The Detailed Chemical Properties of M31 Star Clusters I. Fe, Alpha and Light Elements

TL;DR

This study analyzes the detailed chemical compositions of 31 M31 globular clusters using high-resolution spectra, revealing their ages, metallicities, and element abundances, and providing insights into M31's chemical history and cluster formation.

Contribution

It introduces a novel high-resolution integrated light abundance analysis technique and applies it to M31 clusters, uncovering their chemical diversity and evolutionary clues.

Findings

Range of [Fe/H] within 20 kpc of M31's center

Constant [Fe/H]~ -1.6 in outer halo clusters

Evidence of a young globular cluster (1-5 Gyr) in M31

Abstract

We present ages, [Fe/H] and abundances of the alpha elements Ca I, Si I, Ti I, Ti II, and light elements Mg I, Na I, and Al I for 31 globular clusters in M31, which were obtained from high resolution, high signal-to-noise ratio (SNR$>60$) echelle spectra of their integrated light. All abundances and ages are obtained using our original technique for high resolution integrated light abundance analysis of globular clusters. This sample provides a never before seen picture of the chemical history of M31. The globular clusters are dispersed throughout the inner and outer halo, from 2.5 kpc $<$ R$_{\rm M31}$ $<$ 117 kpc. We find a range of [Fe/H] within 20 kpc of the center of M31, and a constant [Fe/H]$\sim-1.6$ for the outer halo clusters. We find evidence for at least one massive globular cluster in M31 with an age between 1 and 5 Gyr. The alpha-element ratios are generally similar to Milky Way globular cluster and field star ratios. We also find chemical evidence for a late-time accretion origin for at least one cluster, which has a different abundance pattern than other clusters at similar metallicity. We find evidence for star-to-star abundance variations in Mg, Na, and Al in the globular clusters in our sample, and find correlations of Ca, Mg, Na, and possibly Al abundance ratios with cluster luminosity and velocity dispersion, which can potentially be used to constrain globular cluster self-enrichment scenarios. Data presented here were obtained with the HIRES echelle spectrograph on the Keck I Telescope.