Star Clusters in M31: II. Old Cluster Metallicities and Ages from Hectospec Data

TL;DR

This study provides detailed spectroscopic analysis of over 300 M31 globular clusters, revealing their metallicity distribution, ages, and differences from the Milky Way's cluster system, suggesting distinct formation histories.

Contribution

First comprehensive spectroscopic survey of M31 globular clusters using Hectospec, determining metallicities and ages, and highlighting differences from the Galactic globular cluster system.

Findings

M31 globular clusters have a broad metallicity peak at [Fe/H]=-1.

No evidence of intermediate-age massive clusters in M31.

Old clusters show consistent ages across a range of metallicities.

Abstract

We present new high signal-to-noise spectroscopic data on the M31 globular cluster system, obtained with the Hectospec multifiber spectrograph on the 6.5m MMT. More than 300 clusters have been observed at a resolution of 5A. The primary focus of this paper is the determination of mean cluster metallicities, ages and reddenings. Metallicities were estimated using a calibration of Lick indices with [Fe/H] provided by Galactic GCs. The metallicity distribution is not generally bimodal, in strong distinction with the bimodal Galactic globular distribution. Rather, the M31 distribution shows a broad peak, centered at [Fe/H]=-1, suggesting that the cluster systems of M31 and the Milky Way had different formation histories. Ages for clusters with [Fe/H] > -1 were determined using the automatic stellar population analysis program EZ_Ages. We find no evidence for massive clusters in M31 with intermediate ages. Moreover, we find that the mean ages of the old GCs are remarkably constant over about a decade in metallicity (-0.95 < [Fe/H] < 0.0). (abridged)