Modelling of integrated-light spectra from the optical to the
  near-infrared: the globular cluster G280 in M31
  In previous papers, we introduced our method for measuring chemical
abundances from integrated-light spectra of globular clusters and applied it to
a variety of extragalactic star clusters. Here we extend our analysis technique
to the infrared. We simultaneously analyse an optical spectrum of the massive
globular cluster G280 in M31, obtained with the HIRES spectrograph on the Keck
I telescope, and an H-band spectrum obtained with NIRSPEC on Keck II. We
discuss the sensitivity of our results to various modifications of the input
assumptions, such as different line lists and isochrones and the possible
presence of a metallicity spread in G280. When using the most recent version of
the Kurucz line list, we measure iron abundances of [Fe/H]=-0.68+/-0.02 from
the optical spectrum and [Fe/H]=-0.60+/-0.07 from the infrared spectrum. These
values agree well with previous spectroscopic determinations of the metallicity
of G280. While the small difference between the optical and infrared
measurements is insignificant given the uncertainties, it is also consistent
with a metallicity spread similar to that observed in massive GCs such as Omega
Cen and G1, and also hinted at by the colour-magnitude diagram of G280. The
optical and infrared spectra both indicate an alpha-enhancement of about
0.3-0.4 dex relative to solar-scaled abundances, as typically also observed in
Milky Way GCs. It appears that our integrated-light analysis technique also
performs well in the H-band. However, complications due to the presence of
molecular bands and telluric contamination are more severe in the infrared, and
accurate modelling of the coolest giants is more critical.
