Stellar population in early-type galaxies: Further evidence for environmental influences

Abstract

Multifiber and long-slit spectra have been used to study the spectral properties of early-type galaxies in a variety of environments, from the centers of dense clusters to relatively isolated situations. A system of spectral indicators, defined in the wavelength interval λλ4000-4400 A, has been used to analyze the integrated spectra. For five of these spectral indices, systematic differences are found between the integrated spectra of early-type galaxies in the centers of dense clusters and those of E/S0's inhabiting relatively low- density environments. In each spectral index diagram, there is a clear separation between early-type galaxies in low-density environments and metal-rich galactic globular clusters, with the E/S0's in high-density environments situated roughly midway between these two extremes. From the behavior of the galaxies and globular clusters in the spectral index diagrams, constraints can be placed on the nature of the stellar population differences between early-type galaxies in high- and low- density environments. The most striking difference is that the luminosity-weighted mean spectral type of the evolved stars (i.e., giant branch and horizontal branch stars) in E/S0's in low-density environments is inferred to be considerably later type and more metal-enriched than in E/S0's in the central regions of dense clusters. On the other hand, the overall integrated spectral type (i.e., the luminosity-weighted mean spectral type of all of the stars in a galaxy) of early-type galaxies does not vary with environment. To reconcile this apparent conflict, we propose that a substantial intermediate-age population is present in the early-type galaxies found in low-density environments, a population that is considerably reduced or altogether lacking in the early-type galaxies in dense clusters. Hence we conclude that early-type galaxies in dense clusters have experienced a truncated star formation and chemical enrichment history when compared to their counterparts in lower-density surroundings.