Group, field and isolated early-type galaxies – I. Observations and nuclear data

This is the first paper of a series on the investigation of stellar population properties and galaxy evolution of an observationally homogeneous sample of early-type galaxies in groups, field and isolated galaxies. Here we present high signal-to-noise ratio (S/N) long-slit spectroscopy of 86 nearby elliptical and S0 galaxies. Eight of them are isolated, selected according to a rigorous criterion, which guarantees a genuine low-density subsample. The present survey has the advantage of covering a larger wavelength range than normally found in the literature, which includes [O iii]λ5007 and Hα, both lines important for emission correction. Among the 86 galaxies with S/N ≥ 15 (per resolution element, for re/8 central aperture), 57 have their Hβ-index corrected for emission (the average correction is 0.190 Å in Hβ) and 42 galaxies reveal [O iii]λ5007 emission, of which 16 also show obvious Hα emission. Most of the galaxies in the sample do not show obvious signs of disturbances nor tidal features in the morphologies, although 11 belong to the Arp catalogue of peculiar galaxies; only three of them (NGC 750, 751 and 3226) seem to be strongly interacting. We present the measurement of 25 central line-strength indices calibrated to the Lick/IDS system. Kinematic information is obtained for the sample. We analyse the line-strength index versus velocity dispersion relations for our sample of mainly low-density environment galaxies, and compare the slope of the relations with cluster galaxies from the literature. Our main findings are that the index–σ0 relations presented for low-density regions are not significantly different from those of cluster E/S0s. The slope of the index–σ0 relations does not seem to change for early-type galaxies of different environmental densities, but the scatter of the relations seems larger for group, field and isolated galaxies than for cluster galaxies.