Evidence for intermediate-age stellar populations in early-type galaxies from K-band spectroscopy

Abstract

The study of stellar populations in early-type galaxies in different environments is a powerful tool for constraining their star formation histories. This study has been traditionally restricted to the optical range, where dwarfs around the turn-off and stars at the base of the RGB dominate the integrated light at all ages. The near-infrared spectral range is especially interesting since in the presence of an intermediate-age population, AGB stars are the main contributors. In this letter, we measure the near-infrared indices NaI and D${}_{\mathrm{CO}}$ for a sample of 12 early-type galaxies in low density environments and compare them with the Fornax galaxy sample presented by Silva et al. (2008). The analysis of these indices in combination with Lick/IDS indices in the optical range reveals i) the NaI index is a metallicity indicator as good as C4668 in the optical range, and ii) D${}_{\mathrm{CO}}$ is a tracer of intermediate-age stellar populations. We find that low-mass galaxies in low density environments show higher NaI and D${}_{\mathrm{CO}}$ than those located in Fornax cluster, which points towards a late stage of star formation for the galaxies in less dense environments, in agreement with results from other studies using independent methods.

7 pages, 3 figures, accepted for publication in ApJL

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