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

Abstract

Context. 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. Our work so far, however, has concentrated primarily on the optical range 4200 Å-6200 Å. Aims. Here we extend our analysis technique to the infrared and test it on an H-band spectrum of the massive globular cluster G280 in M31. Methods. We simultaneously analyse an optical spectrum of G280, 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. Results. 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 ω Cen and G1, and also hinted at by the colour-magnitude diagram of G280. The optical and infrared spectra both indicate an α-enhancement of about 0.3-0.4 dex relative to solar-scaled abundances, as typically also observed in Milky Way GCs. Conclusions. From this analysis, 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.

The Massive M31 Cluster G1: Detailed Chemical Abundances from Integrated Light Spectroscopy

“…Finally, it is worth noting that chemically G1 is also very similar to B225, the other massive cluster that has been studied at high spectral resolution, except that B225 is slightly more metal-rich (Colucci et al 2014;Larsen et al 2018b). There are several additional unstudied, massive GCs in the inner regions, many of which appear to be similarly metal-rich.…”

“…This figure demonstrates that the higher mass, higher velocity dispersion clusters tend to have higher IL [Na/Fe] ratios. G1's high [Na/Fe] is very similar to B225, another massive cluster (Larsen et al 2018b). This trend with mass is also reinforced by observations of Milky Way GCs, where more massive clusters seem to have relatively fewer of the "primordial" population stars (Milone et al 2017).…”

“…A handful of these GCs (B023, B158, and B225) have been photometrically identified as candidate iron-complex clusters, all of which were found to have [Fe/H] ∼ − 0.9 to −0.6 (Fuentes-Carrera et al 2008). Colucci et al (2014), Larsen et al (2018b) have all conducted high-resolution IL spectroscopic analyses of B225, finding [Fe/H] ∼ −0.6. and Larsen et al (2018b) further found minimal offsets between the optical and infrared [Fe/H] ratios, placing constraints on the extent of an iron spread.…”

“…Colucci et al (2014), Larsen et al (2018b) have all conducted high-resolution IL spectroscopic analyses of B225, finding [Fe/H] ∼ −0.6. and Larsen et al (2018b) further found minimal offsets between the optical and infrared [Fe/H] ratios, placing constraints on the extent of an iron spread. Another massive M31 GC, B088, is more metalpoor ([Fe/H] = −1.7; , similar to ω Cen; suggested that variations in [Mg/Fe] between the optical and infrared could indicate an iron spread within B088.…”

Chemical composition of globular clusters in dwarf galaxies

“…In Fig. 2 we present updated data for the α-elements ([Ca/Fe], [Ti/Fe]), with additional observations of 3 GCs in NGC 185, two in NGC 205, and one GC in M31 (Larsen et al (2018b)). Symbols are coloured according to the host galaxies of the clusters: red circles mark GCs in dwarf galaxies, blue squares GCs in M33/M31, and black squares our integrated-light observations of Milky Way GCs (Larsen et al 2017).…”