Cluster versus field elliptical galaxies and clues on their formation

Using new observations for a sample of 931 early-type galaxies, we investig ate whether the Mg-2-sigma(0) relation shows any dependence on the local en vironment. The galaxies have been assigned to three different environments depending on the local overdensity (clusters, groups, and field); we used o ur complete redshift database to guide the assignment of galaxies. It is fo und that cluster, group, and field early-type galaxies follow almost identi cal Mg-2-sigma(0) relations, with the largest Mg-2 zero-point difference (c lusters minus field) being only 0.007 +/- 0.002 mag. No correlation of the residuals is found with the morphological type or the bulge-to-disk ratio. Using stellar population models in a differential fashion, this small zero- point difference implies a luminosity-weighted age difference of only simil ar to 1 Gyr between the corresponding stellar populations, with field galax ies being younger. The mass-weighted age difference could be significantly smaller if minor events of late star formation took place preferentially in field galaxies. We combine these results with the existing evidence for th e bulk of stars in cluster early-type galaxies having formed at very high r edshift and conclude that the bulk of stars in galactic spheroids had to fo rm at high redshifts (z greater than or similar to 3), no matter whether su ch spheroids now reside in low- or high-density regions. The cosmological i mplications of these findings are briefly discussed.