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1 Introduction

N-body simulations of CDM models predict that early-type galaxies now residing into clusters and groups formed through major merging at redshift $z\approx 3$, while those formed later than $z\approx 1$ have not yet become virialized structures (Governato et al. 1997 and reference therein). Hierarchical clustering scenarios then suggest that early-type galaxies we see today in clusters and in low density environments have had different histories. Cluster objects are not only ancient but are predicted to have homogeneous properties. Early-type galaxies in low density environments are expected to be younger than the cluster counterparts and more heterogeneous (e.g. most effective galaxy-galaxy interactions take place there). They are then currently interpreted as the final product of processes involving major/minor merging events (see e.g. reviews of Schweizer 1992 and Barnes 1997), but evidences accumulate that both various degrees of dissipation mechanisms (Bender 1997) and weak-interactions (Thomson 1991) have played a significant role in determining the final structure.

Photometric and structural observations of early-type galaxies support hierarchical clustering scenarios showing an environmental dependence. The detection of fine structures and of peculiar kinematics, considered evidence of recent merging/acquisition events, is significantly more frequent among the isolated galaxies then among the cluster members (Malin & Carter 1983; Schweizer 1992; Reduzzi et al. 1996). The homogeneity of the population of early-type galaxies in clusters is otherwise supported by the Fundamental Plane correlations, examined for several clusters, which may be considered universal to 10%-20% (Djorgovski et al. 1995).

Recently, more efforts, both in the theoretical and in the observational fields, have been dedicated to the determination of sound age indicators for early-type galaxies. Line-strength indices have been used to infer the star formation (SF) history of early-type galaxies (see Davies 1995 and references therein), crucial to shed light on the above picture, but firm conclusions have not yet been reached. Buzzoni et al. (1992) suggest that early-type galaxies in the field are almost coeval and characterized by an old stellar population with an average age of $\approx$ 12 Gyr (only 2% of the objects are younger than 5 Gyr). Gonzalez (1993; G93) explains the distribution in the (log[MgFe]-logH$\beta$) plane of a sample of 41 "normal'' ellipticals (a part of which belongs to clusters e.g. Virgo) as due to quite large age differences (from 4 Gyr to 15 Gyr). Rose (1995) suggests that star formation terminated at an earlier epoch in early-type galaxies in the central regions of dense clusters then in objects in lower density environments. Caveats are raised by Charlot & Silk (1994) and Bressan et al. (1996) who remark that some line-strength indicators give contradictory evidences about the age of the stellar populations and that the IMF could depend on the galaxy mass. Jorgensen & Franx (1995) indicate that possible star bursts could be the reason of the age differences among cluster members which contribute significantly to the scatter in the FP. In a recent review on our current understanding of star formation in early-type galaxies, Bender et al. (1996) suggest that the bulk of the stars in the luminous cluster ellipticals must be old and coeval, while the low luminosity early-type galaxies could have smaller ages or extended star formation histories. Bender et al. also suggest that the early-type galaxies in low density environments could be genuinely younger than the luminous cluster ellipticals.

This paper is the first of a series dedicated to the study of a typical population of early-type galaxies in low density environments of galaxies showing present/past signs of interaction. We aim at obtaining information about galaxy formation by investigating their SF history, and the role of the interaction in their evolution. A comparison with the results obtained by other authors in denser environments (Burstein et al. 1984; Pickles 1985; Rose 1985; Rose 1994; Bower et al. 1990; Gonzalez 1993) will further allow us to consider the dependence of the environment on the formation/evolution processes of the early-type galaxies.

The present paper presents the results obtained from the analysis of the spectrophotometric data. It is organized as follows. In Sect. 2 the sample and its characteristics are outlined. Observations and preliminary reduction are presented in Sect. 3. Indices definitions and procedures followed to measure them are given in Sect. 4, where a detailed description of the procedures adopted to transform our measures of the "red'' indices in the Lick-IDS "standard'' system is also given. Comparisons with stellar and galactic data taken from Worthey (1996, private communication) and from G93 show the final consistency between the two measurements systems. Preliminary conclusions are reported in Sect. 5. In the appendix we present the data relative to a set of hot stars (T>10000 K) suitable to extend the Worthey (1992; W92) fitting functions.

Further papers will present kinematical information (Longhetti et al. 1997a, Paper II) and ad hoc spectrophotometric models (Longhetti et al. 1997b, Paper III; Longhetti et al. 1998, Paper IV) developed in order to calibrate new spectral indices. These latter will be useful to infer from line-strength indices the presence and the strength of secondary SF events induced by the interaction.


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Up: Star formation history of environments

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