Group, field and isolated early-type galaxies -: II.: Global trends from nuclear data

We have derived ages, metallicities and enhanced-element ratios [alpha/Fe] for a sample of 83 early-type galaxies essentially in groups, the field or isolated objects. The stellar-population properties derived for each galaxy correspond to the nuclear r(e)/8 aperture extraction. The median age found for Es is 5.8 +/- 0.6 Gyr and the average metallicity is +0.37 +/- 0.03 dex. For S0s, the median age is 3.0 +/- 0.6 Gyr and [Z/H] = 0.53 +/- 0.04 dex. We compare the distribution of our galaxies in the H beta-[MgFe] diagram with Fornax galaxies. Our elliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster. We find that the galaxies lie in a plane defined by [Z/H] = 0.99 log sigma(0) - 0.46 log(age) - 1.60, or in linear terms Z alpha sigma(0) x (age) (- 0.5). More massive (larger sigma(0)) and older galaxies present, on average, large [alpha/Fe] values, and therefore must have undergone shorter star-formation time-scales. Comparing group against field/isolated galaxies, it is not clear that environment plays an important role in determining their stellar-population history. In particular, our isolated galaxies show ages differing by more than 8 Gyr. Finally we explore our large spectral coverage to derive log (O/H) metallicity from the H alpha and N (II) lambda 6584 and compare it with model-dependent [Z/H]. We find that the O/H abundances are similar for all galaxies, and we can interpret it as if most chemical evolution has already finished in these galaxies.