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arXiv · 1708.04638

The puzzling interpretation of NIR indices: The case of NaI2.21

B. Röck·A. Vazdekis·F. La Barbera·R. F. Peletier·J. H. Knapen·C. Allende-Prieto·D. S. Aguado
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Abstract

We present a detailed study of the Na I line strength index centered in the $K$-band at $22100$, {\AA} (NaI2.21 hereafter) relying on different samples of early-type galaxies. Consistent with previous studies, we find that the observed line strength indices cannot be fit by state-of-art scaled-solar stellar population models, even using our newly developed models in the NIR. The models clearly underestimate the large NaI2.21 values measured for most early-type galaxies. However, we develop a Na-enhanced version of our newly developed models in the NIR, which - together with the effect of a bottom-heavy initial mass function - yield NaI2.21 indices in the range of the observations. Therefore, we suggest a scenario in which the combined effect of [Na/Fe] enhancement and a bottom-heavy initial mass function are mainly responsible for the large NaI2.21 indices observed for most early-type galaxies. To a smaller extent, also [C/Fe] enhancement might contribute to the large observed NaI2.21 values.

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B. Röck, A. Vazdekis, F. La Barbera, R. F. Peletier, J. H. Knapen, C. Allende-Prieto, D. S. Aguado. 2017-08-15. The puzzling interpretation of NIR indices: The case of NaI2.21. https://doi.org/10.1093/mnras%2Fstx1940

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