Near-infrared spectroscopic indices for unresolved stellar populations – II. Index measurements

We measured the equivalent width of a large set of near-infrared (NIR, 0.8–2.4 μm) line-strength indices in XShooter medium-resolution spectra of the central regions of 14 galaxies. We found that two aluminum indices, Al at 1.31 μm and Al1 at 1.67 μm, and two CO indices, CO1 at 1.56 μm and CO4 at 1.64 μm, are tightly correlated with the velocity dispersion. Moreover, the NIR Al and CO1 indices show strong correlations with the optical Mg2 and Mgb indices, which are usually adopted as α/Fe-enhancement diagnostics. The molecular FeH1 index at 1.58 μm correlates tightly with the optical 〈Fe〉 and [MgFe]′ indices, which are used as total metallicity diagnostics. The NIR Paβ index at 1.28 μm has a behaviour similar to the optical Hβ index, which is a diagnostic of mean age. We defined two new composite indices,〈Al〉 and [AlFeH], as possible candidates to be used as NIR diagnostics of total metallicity and α/Fe enhancement. The NIR〈Al〉 index has a strong correlation with the optical Mg2 and Mgb indices, while the [AlFeH] index is tightly correlated with the optical〈Fe〉and [MgFe]′ indices. The distribution of the data points in the NIR Paβ–〈Al〉and Paβ–[AlFeH] diagrams mimics that in the optical [MgFe]′–Hβ and Mgb–〈Fe〉diagrams, which are widely used to constrain the properties of the unresolved stellar populations. We concluded that some NIR line-strength indices could be useful in studying stellar populations as well as in fine-tuning stellar population models.