Evidence for intermediate-age stellar populations in early-type galaxies
  from K-band spectroscopy
  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$_{\rm CO}$ 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$_{\rm CO}$ is a
tracer of intermediate-age stellar populations. We find that low-mass galaxies
in low density environments show higher NaI and D$_{\rm CO}$ 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.
