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

© ESO 2006. We would like to thank Brad Gibson, Daisuke Kawata and Christopher Thom for their careful reading of the manuscript and for their useful comments. We are also grateful to Javier Cenarro, Reynier Peletier, and Alexandre Vazdekis for fruitful discussions. We are very grateful to the referee, Jim Rose, for his very constructive report and many useful suggestions. The WHT is operated on the island of La Palma by the Royal Greenwich Observatory at the Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. The Calar Alto Observatory is operated jointly by the Max-Planck Institute für Astronomie, Heidelberg, and the Spanish Instituto de Astrofísica de Andalucía (CSIC). This research has made use of the SIMBAD database operated at CDS, Strasbourg, France. This work was supported by the Spanish research project AYA 2003-01840, the European Community through its NOVA fellowship program, and the Australian Research Council. We are grateful to the CATs for generous allocation of telescope time.
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.
Spanish research project
European Community (NOVA fellowship program)
Australian Research Council
Depto. de Física de la Tierra y Astrofísica
Fac. de Ciencias Físicas
TRUE
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