1 Introduction

The stellar populations of early-type galaxies exhibit very homogeneous properties. This evidence comes from the Fundamental Plane (Djorgovski & Davis 1987; Dressler et al. 1987; Bender et al. 1992; Jørgensen et al. 1993; Renzini & Ciotti 1993; Saglia et al. 1993) and a number of scaling relations whose main characteristics are small internal scatters. Besides a well-defined correlation between color and magnitude (Bower et al. 1992), the Mg-$\sigma$ correlation (Bender et al. 1993) linking the stellar populations with the kinematical parameters of early-type galaxies also appears narrow. The studies mentioned above find that for bright cluster ellipticals the scatter in age is less than 15 per cent. This conclusion gets further support from the evolution of the above relations with redshift for cluster galaxies (Aragon-Salamanca et al. 1993; Dickinson 1995; Bender et al. 1996; Ziegler & Bender 1997; Bender et al. 1998; van Dokkum et al. 1998; Jørgensen et al. 1999; Saglia et al. 2000).

The above findings are restricted to dense cluster environments. As theories of hierarchical galaxy formation predict that elliptical galaxies in the field will have less homogeneous properties and will be on average younger than their counterparts in clusters (e.g., Kauffmann et al. 1993; Cole et al. 1994; Thomas & Kauffmann 1999, and references therein), it is particularly important to study early-type galaxies in different environments. Jørgensen et al. (1996) investigated the Fundamental Plane (FP) in 11 clusters of galaxies and found a small scatter of about 20 per cent. Moreover, they could not gather any evidence for differences in the FP depending on the environment. Also when leaving the clusters and going to less dense environments there is only weak and not yet compelling evidence that the homogeneity might disappear. Larson et al. (1980) find that the scatter of the color-magnitude relation in the field is slightly larger than that for cluster ellipticals. De Carvalho & Djorgovski (1992) derive a larger scatter for the FP in the field compared to clusters. On the other hand, analyzing spectral features in the range $\lambda = 4000$-4400 Å, Rose et al. (1994) find that "the overall spectral type [...] of early-type galaxies does not vary with the environment''. Interestingly, James & Mobasher (1999), investigating the CO absorption feature at 2.3 mm, find evidence for intermediate-age population only for galaxies in small groups, while the isolated early-type galaxies of their sample form a very uniform old population.

A further powerful tool to study stellar population properties of elliptical galaxies are measurements of absorption line-strength indices. In particular, a combination of age- and metallicity-sensitive indices allows for the derivation of average ages, metallicities, and $\alpha$/Fe ratios (Worthey et al. 1992; Trager et al. 2000). Such line-strength indices are measured mostly for cluster galaxies in a number of studies in the literature (Peletier 1989; Worthey et al. 1992; González 1993; Davies et al. 1993; Carollo & Danziger 1994; Fisher et al. 1995; Kuntschner 2000; Mehlert et al. 1998, 2000; Jørgensen 1999; Poggianti et al. 2001). In these studies, although the samples show a larger spread in ages than what is inferred from the scaling relations, (in particular massive) elliptical galaxies are found to have old mean ages. It should also be noted that composite stellar population models with a small metal-poor subcomponent can reproduce the Balmer absorption of all massive elliptical galaxies without invoking young ages (Maraston & Thomas 2000). A further interesting result derived from the Mg- and Fe-indices are the super-solar $\alpha$/Fe ratios, implying short star formation timescales (e.g., Matteucci 1994; Thomas et al. 1999).

Most of the ellipticals for which absorption line-strength indices are measured are in clusters. Our goal is to fill this gap in the literature, and present in this paper a sample of 148 nearby early-type galaxies which were observed spectroscopically in the wavelength range of $\lambda \simeq 4450$-6650 Å. The following Lick/IDS absorption indices were measured: H${\beta}$, Mg₁, Mg₂, Mgb, Fe5015, Fe5406, Fe5270, Fe5335, Fe5709, Fe5782, NaD, TiO₁ and TiO₂. Their wavelength definitions can be found in Worthey et al. (1994). This paper presents the data which form the basis of our investigations.

The paper is organized as follows. Section 2 describes the sample properties. Section 3 explains the observations and the data reduction. The presence of emission is investigated in Sect. 4, followed by a summary in Sect. 5.