Spectroscopic constraints on the stellar population of elliptical galaxies in the Coma cluster

Near-infrared spectra for a sample of 31 elliptical galaxies in the Coma cluster are obtained. The galaxies are selected to be ellipticals (no lenticulars), with a large spatial distribution, covering both the core and outskirts of the cluster (i.e. corresponding to regions with large density contrasts). COsp (2.3-mu m) absorption indices, measuring the contribution from intermediate-age red giant and supergiant stars to the near-infrared light of the ellipticals, are then estimated. It is found that the strength of COsp features in elliptical galaxies increases from the core (r < 0 degrees.2) to the outskirts (r > 0 degrees.2) of the Coma cluster. Using the Mg-2 strengths, it is shown that the observed effect is not caused by metallicity and is mostly caused by the presence of a younger population (giant and supergiant stars) in ellipticals in the outskirts (low-density region) of the cluster. Using the COsp features, the origin of the scatter on the near-infrared Fundamental Plane (the relation between the effective diameter, effective surface brightness and velocity dispersion) of elliptical galaxies is studied. Correcting this relation for contributions from the red giant and supergiant stars, the rms scatter reduces from 0.077 td 0.073 dex. Although measurable, the contribution from these intermediate-age stars to the scatter on the near-infrared Fundamental Plane of ellipticals is only marginal. A relation is found between the COsp and V - K colours of ellipticals, corresponding to a slope of 0.036 +/- 0.016, significantly shallower than that from the Mg-2-(V - K) relation. This is studied using stellar synthesis models.