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

We have derived ages, metallicities and enhanced-element ratios [α/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 re/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 Hβ-[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 σ0− 0.46 log(age) − 1.60, or in linear terms Z ∝σ0× (age) −0.5. More massive (larger σ0) and older galaxies present, on average, large [α/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 Hα and N iiλ6584 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.