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

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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 red giant branch dominate the integrated light at all ages. The near-infrared spectral range is especially interesting since in the presence of an intermediate-age population, asymptotic giant branch stars are the main contributors. In this Letter, we measure the near-infrared indices Na I and D(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.. The analysis of these indices in combination with Lick/IDS indices in the optical range reveals that (1) the Na I index is a metallicity indicator as good as C4668 in the optical range, and (2) D(CO) is a tracer of intermediate-age stellar populations. We find that low-mass galaxies in low-density environments show higher Na I and D(CO) than those located in the Fornax cluster, which points toward a late stage of star formation for the galaxies in less dense environments, in agreement with results from other studies using independent methods.
© 2009. The American Astronomical Society. The authors acknowledge the referee, Bahram Mobasher, for his useful comments. E. M.-Q. acknowledges the SMES for a FPI PhD fellowship. This work has been partially supported by the SMES through the research project AYA2007-67752-C0301. N. C. also acknowledges financial support from the research project AYA2006-15698-C02-02. P. S.-B. acknowledges the financial support from a Marie Curie European Reintegration Grant within the 7th European Community Framework Programme.
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