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 asample of 98 early-type galaxies in different environments. For most indices(Mgb and in particular) ellipticals in rich clusters and in low-densityregions follow the same index-sigma relations. However, striking spectraldifferences between field ellipticals and their counterparts in the centralregion of the Coma cluster are found for the first time, with galaxies in thedenser environment showing significantly lower C4668 and CN2 absorptionstrengths. The most convincing interpretation of these results is a differencein abundance ratios, arising from a distinct star formation and chemicalenrichment histories of galaxies in different environments. An scenario inwhich elliptical galaxies in clusters are fully assembled at earlier stagesthan their low-density counterparts is discussed.Comment: 12 pages, including 3 figures, accepted for publication in ApJ