Stellar population of elliptical galaxies in different environments: Spectroscopic CO observations

Near-infrared spectra of a sample of 21 elliptical galaxies in the Pisces and Abell 2634 clusters and the general field have been obtained. The strengths of CO (2.3 mu m) absorption features in these galaxies are used to explore the presence of an intermediate-age population (e.g., asymptotic giant branch stars) in ellipticals in different environments. The CO absorption features in the spectra of field ellipticals are found to be stronger than those in clusters, with average indices of 0.247 +/- 0.018 and 0.218 +/- 0.018 respectively. The two distributions are different at the 95 per cent confidence level. Differences in metallicities and absolute magnitudes among the field and cluster ellipticals cannot explain these observations. This is interpreted as evidence for an intermediate-age stellar population in the field galaxies, indicative of a more extended or recent epoch of star formation. The effect of CO indices on the scatter in the optical D-n-sigma and L-sigma relations of ellipticals is explored, and there are indications of a smaller intrinsic scatter in these relations for isolated galaxies compared to those in richer environments.