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

TL;DR

This paper uses near-infrared spectroscopy to identify intermediate-age stellar populations in early-type galaxies, revealing environmental effects on their star formation histories.

Contribution

It demonstrates that near-infrared indices NaI and D$_{ m CO}$ effectively trace metallicity and intermediate-age stars, providing new tools for galaxy population analysis.

Findings

NaI is a strong metallicity indicator.

D$_{ m CO}$ traces intermediate-age populations.

Low-mass galaxies in low-density environments show signs of recent star formation.

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$_{\rm CO}$ 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.