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

Aguado, D. S.; Allende-Prieto, C.; Knapen, J. H.; La Barbera, F.; Peletier, R. F.; Vazdekis, A.; Röck, B.
Bibliographical reference

Monthly Notices of the Royal Astronomical Society, Volume 472, Issue 1, p.361-372

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We present a detailed study of the Na I line strength index centred in the K band at 22 100 Å (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-the-art scaled-solar stellar population models, even using our newly developed models in the near infrared (NIR). The models clearly underestimate the large NaI2.21 values measured for most early-type galaxies. However, we develop an 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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