Differences in Carbon and Nitrogen Abundances between Field and Cluster Early-Type Galaxies

Central line-strength indices were measured in the blue spectral region for a sample of 98 early-type galaxies in different environments. For most indices (Mg b and <Fe> in particular), elliptical galaxies in rich clusters and in low-density regions follow the same index-σ relations. However, striking spectral differences between field elliptical galaxies and their counterparts in the central region of the Coma Cluster are found for the first time, with galaxies in the denser environment showing significantly lower C4668 and CN₂ absorption strengths. The most convincing interpretation of these results is that they represent a difference in abundance ratios arising from distinct star formation and chemical-enrichment histories of galaxies in different environments. A scenario in which elliptical galaxies in clusters are fully assembled at earlier stages than their low-density counterparts is discussed.