Stellar populations of early-type galaxies in different environments - I. Line-strength indices. Relations of line-strengths with sigma

Aims. This paper commences a series devoted to the study of the stellar content of early-type galaxies. The goal of the series is to set constraints on the evolutionary status of these objects. Methods. In this paper we describe the details of the galaxy sample, the observations, and the data reduction. Line-strength indices and velocity dispersions (sigma) are measured in 98 early-type galaxies drawn from different environments, and the relation of the indices with the velocity dispersion analysed in detail. Results. The present sample indicates that some of the index-sigma relations depend on galaxy environment. In particular, the slope of the relation between Balmer lines and s is steeper for galaxies in the Virgo cluster, small groups, and in the field than for galaxies in the Coma cluster. In several indices there is also a significant sigma. set in the zero point between the relations defined by the different subsamples. The slopes of the index-sigma relation for the Virgo and low-density environment galaxies are explained by a variation of both age and metallicity, with velocity dispersion, as previously noted in other studies. For the galaxies in the Coma cluster, however, the relation of the indices with s only requires a variation of the abundance along the s sequence. In agreement with other studies, we find that the models that better reproduce the slopes are those in which the a elements vary more than the Fe-peak elements along the s sequence, while, at a given s, older galaxies show an higher sigma/Fe ratio. Conclusions. The results can be explained assuming that galaxies in the Coma cluster have experienced a truncated star formation and chemical enrichment history compared to a more continuous time-extended history for their counterparts in lower density environments