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

Abstract

We measured the equivalent width of a large set of near-infrared (NIR, 0.8--2.4$\hat{I}¼$m) line-strength indices in the XShooter medium-resolution spectra of the central regions of 14 galaxies. We found that two aluminum indices Al at 1.31 $\hat{I}¼$m and Al1 at 1.67 $\hat{I}¼$m and the two CO indices CO1 at 1.56 $\hat{I}¼$m and CO4 at 1.64 $\hat{I}¼$m 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 $\hat{I}\pm$/Fe-enhancement diagnostics. The molecular FeH1 index at 1.58 $\hat{I}¼$m tightly correlates with the optical <Fe> and [MgFe]' indices, which are used as total metallicity diagnostics. The NIR Pa${\hat{I}}^2$ index at 1.28 $\hat{I}¼$m has a behaviour similar to the optical H${\hat{I}}^2$ 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 $\hat{I}\pm$/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 Pa${\hat{I}}^2$-<Al> and Pa${\hat{I}}^2$-[AlFeH] diagrams mimic that in the optical [MgFe]'-H${\hat{I}}^2$ and the Mgb-<Fe> diagrams, which are widely used to constraint 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.

15 pages, 9 figures. Accepted for publication in MNRAS