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] lambda5007 and Halpha, both lines important for emission correction. Among the 86 galaxies with S/N greater than or equal to 15 (per resolution element, for r(e)/8 central aperture), 57 have their H-index corrected for emission (the average correction is 0.190 Angstrom in Hbeta) and 42 galaxies reveal [O III] lambda5007 emission, of which 16 also show obvious H 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-sigma(0) relations presented for low-density regions are not significantly different from those of cluster E/S0s. The slope of the index-sigma(0) 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.