Near-infrared integrated spectra of Galactic globular clusters : testing simple stellar population models

Abstract

We present SOAR/OSIRIS cross-dispersed near-infrared (NIR) integrated spectra of 12 Galactic globular clusters that are employed to test Maraston NIR evolutionary population synthesis models, and to provide spectral observational constraints to calibrate future models.We measured equivalent widths (Wλ) of the most prominent NIR absorption features: λ 1.49 μm, MgI λ 1.58 μm, Fe I/Mg I, λ 1.59 μm, Si I, λ 1.71 μm, MgI, λ 2.21 μm, Na I and λ 2.26 μm, Ca I as well as the λ 1.62 μm, λ 2.29 μm, CO and λ 2.05 μm, CN molecular bands. Optical Wλ of G band (4300 Å), Hβ, Mg2, Fe I (4531, 527 and 5335 Å) and Na I (5897 Å) were also measured. The globular clusters Wλ were compared with model predictions with ages within 4–15 Gyr, and metallicities between 0.005 and 2 Zʘ . Observed integrated colours (B − V, V − I and V − Ks) were also compared with models. The NIR integrated spectra among our sample appear qualitatively similar in most of the absorption features. The Maraston models can properly predict the optical Wλ observed in globular clusters. Regarding the NIR, they do underestimate the strength of MgI 1.49 μm, but they can reproduce the observed Wλ of Fe I 1.58 μm, Si I 1.59 μm and CO 2.29 μm, in about half of our sample. The remaining objects require the inclusion of intermediate-age populations. Thus, we suggest that the presence of Cand O-rich stars in models is important to reproduce the observed strengths of metallic lines. Another possibility is the lack of α-enhancement in the models. In the case of the optical and NIR Fe I lines, standard models and those that include blue horizontal branch stars produce similar results. A similar trend is observed for Na I 5895 Å, while in the case of the G band, the models with blue horizontal branch do describe better the observations. For most of the sample, the optical to NIR colours are well described by the Maraston models. In general, Maraston models can provide reliable information on the NIR stellar population of galaxies, but only when Wλ and colours are taken together; in other words, Wλ and continuum fluxes should be simultaneously fitted. However, the results should be taken with caution, since the models tend to predict results biased towards young ages.