Stellar Population of Ellipticals in Different Environments: Near-infrared Spectroscopic Observations

Near-infrared spectra of 50 elliptical galaxies in the Pisces, A2199 and A2634 clusters, and in the general field, have been obtained. The strength of the CO (2.3 micron) absorption feature in these galaxies is used to explore the presence of an intermediate-age population (e.g. Asymptotic Giant Branch stars) in ellipticals in different environments. We find the strongest evidence for such a population comes from ellipticals in groups of a few members, which we interpret as the result of recent minor merging of these galaxies with later type galaxies. Field galaxies from very isolated environments, on the other hand, show no evidence for young or intermediate-age stars as revealed by H beta and CO absorptions, and appear to form a very uniform, old population with very little scatter in metallicity and star formation history.