The epochs of early-type galaxy formation

By means of population synthesis models with variable alpha/Fe ratios we derive average ages, metallicities, and [alpha/Fe] element enhancements for a sample of 126 field and cluster early-type galaxies. We find a clear positive relation between [alpha/Fe] and velocity dispersion. Zero-point, slope, and scatter of this correlation are the same for cluster and field galaxies. In particular, the [alpha/Fe] ratios and mean ages of cluster ellipticals are positively correlated. This strongly reinforces the view that the [alpha/Fe] element enhancement in ellipticals is produced by star formation timescales rather than by variations of the initial mass function. These results indicate that the more massive the galaxy, the shorter is its star formation timescale, and the higher is the redshift of the bulk of star formation. This finding is not compatible with the predictions from models of hierarchical galaxy formation. The lenticular and field galaxies of the investigated sample do not follow the correlation between age and [alpha/Fe], but contain a non-negligible fraction of galaxies with young average ages and high [alpha/Fe] ratios.