Cluster versus field elliptical galaxies and clues on their formation

Using new observations for a sample of 931 early-type galaxies, we investigate whether the Mg₂-σ₀ relation shows any dependence on the local environment. The galaxies have been assigned to three different environments depending on the local overdensity (clusters, groups, and field); we used our complete redshift database to guide the assignment of galaxies. It is found that cluster, group, and field early-type galaxies follow almost identical Mg₂-σ₀ relations, with the largest Mg₂ zero-point difference (clusters 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 ∼1 Gyr between the corresponding stellar populations, with field galaxies 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 the bulk of stars in cluster early-type galaxies having formed at very high redshift and conclude that the bulk of stars in galactic spheroids had to form at high redshifts (z ≳ 3), no matter whether such spheroids now reside in low- or high-density regions. The cosmological implications of these findings are briefly discussed.