TITLE: The SLUGGS Survey: Calcium Triplet-based Spectroscopic Metallicities for Over 900 Globular Clusters
AUTHORS: Christopher Usher et al.
FIRST AUTHOR’S INSTITUTION: Center for Astrophysics and Supercomputer, Swinburne University of Technology
In the case of globular clusters, astronomers use a color-metallicity relationship. They measure the color of a cluster, which is more easily obtained via photometry, and from this formula they can derive a metallicity. However, because many color-metallicity relationships have flat intermediate regions – areas where a small change in underlying metallicity equates to a large change in color – it has been shown that a unimodal metallicity distribution can appear bimodal. The SLUGGS team uses a different proxy: the calcium triplet.The calcium triplet consists of three atomic lines at 8498 A, 8542 A, and 8662 A. It’s one of the strongest stellar spectral features in the optical/NIR, and its strength increases with metallicity. They first calibrate a relationship between the strength of the calcium triplet and the metallicity of a cluster, then use this relationship to explore whether the observed bimodal distribution in metallicity of globular clusters is real or an artifact of the color-metallicity formula.The SLUGGS team assembled the largest spectroscopic globular cluster metallicity dataset yet analyzed, composed of 903 globular clusters around 11 early-type galaxies. Eight of these galaxies had data of sufficiently high quality to be useful, and in six of these galaxies the team finds metallicity bimodality. This agrees with the color-metallicity results, and implies that most massive galaxies have undergone multiple epochs of star formation. However, there are still some issues with the relation – in half of the galaxies sampled, the distribution of calcium triplet-based metallicities did not agree with the color-metallicity distribution at either the low or high end of the metallicity range. A larger sample of extragalactic globular clusters will be required before the question can be answered with certainty.