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