The SLUGGS survey: Globular cluster stellar population trends from weak absorption lines in stacked spectra

TL;DR

This study uses stacked spectra of globular clusters from multiple galaxies to analyze stellar population variations, revealing differences in metallicity relations and potential age or abundance variations across galaxies.

Contribution

It provides new insights into globular cluster stellar populations by examining weak absorption lines and their relation to galaxy environment and properties.

Findings

Metal line strengths vary at fixed colour and magnitude.

Color-metallicity relations differ between galaxies.

Brighter clusters tend to be redder with stronger metal lines.

Abstract

As part of the SLUGGS survey, we stack 1137 Keck DEIMOS spectra of globular clusters from 10 galaxies to study their stellar populations in detail. The stacked spectra have median signal to noise ratios of $\sim 90$ \AA$^{-1}$. Besides the calcium triplet, we study weaker sodium, magnesium, titanium and iron lines as well as the H$\alpha$ and higher order Paschen hydrogen lines. In general, the stacked spectra are consistent with old ages and a Milky Way-like initial mass function. However, we see different metal line index strengths at fixed colour and magnitude, and differences in the calcium triplet--colour relation from galaxy to galaxy. We interpret this as strong evidence for variations in the globular cluster colour--metallicity relation between galaxies. Two possible explanations for the colour--metallicity relation variations are that the average ages of globular clusters vary from galaxy to galaxy or that the average abundances of light elements (i.e. He, C, N and O) differ between galaxies. Stacking spectra by magnitude, we see that the colours become redder and metal line indices stronger with brighter magnitudes. These trends are consistent with the previously reported `blue tilts' being mass--metallicity relations.