The SLUGGS Survey: Calcium Triplet-based Spectroscopic Metallicities for Over 900 Globular Clusters

TL;DR

This study presents the largest spectroscopic metallicity catalog for globular clusters in 11 early-type galaxies, revealing bimodal metallicity distributions that support two-phase star formation histories.

Contribution

Redefines the calcium triplet-metallicity relation and applies it to derive metallicities for over 900 globular clusters, expanding the sample size significantly.

Findings

Most massive early-type galaxies have bimodal metallicity distributions.

Good agreement between spectroscopic metallicities and Lick index estimates.

Bimodality indicates two distinct star formation episodes.

Abstract

Although the colour distribution of globular clusters in massive galaxies is well known to be bimodal, the spectroscopic metallicity distribution has been measured in only a few galaxies. After redefining the calcium triplet index-metallicity relation, we use our relation to derive the metallicity of 903 globular clusters in 11 early-type galaxies. This is the largest sample of spectroscopic globular cluster metallicities yet assembled. We compare these metallicities with those derived from Lick indices finding good agreement. In 6 of the 8 galaxies with sufficient numbers of high quality spectra we find bimodality in the spectroscopic metallicity distribution. Our results imply that most massive early-type galaxies have bimodal metallicity, as well as colour, distributions. This bimodality suggests that most massive galaxies early-type galaxies experienced two periods of star formation.