Modelling of integrated-light spectra from the optical to the near-infrared: the globular cluster G280 in M31

TL;DR

This study extends integrated-light spectral analysis of globular clusters to the infrared, demonstrating consistent metallicity and alpha-element enhancement measurements for G280 in M31 using optical and H-band spectra.

Contribution

We developed and validated an integrated-light spectral analysis method that works effectively in the infrared, complementing optical techniques for globular cluster characterization.

Findings

Optical and infrared spectra yield consistent [Fe/H] measurements.

Alpha-enhancement of about 0.3-0.4 dex observed in G280.

Infrared analysis is feasible but more affected by molecular bands and telluric contamination.

Abstract

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.