zCOSMOS 10k-bright spectroscopic sample Exploring mass and environment dependence in early-type galaxies

Aims. We present the analysis of the U − V rest-frame color distribution and some spectral features as a function of mass and environment for a sample of early-type galaxies up to z = 1 extracted from the zCOSMOS spectroscopic survey. This analysis is used to place constraints on the relative importance of these two properties in controlling galaxy evolution. Methods. We used the zCOSMOS 10k-bright sample, limited to the AB magnitude range 15 < I < 22.5, from which we extracted two different subsamples of early-type galaxies. The first sample (“red galaxies”) was selected using a photometric classification (2098 galaxies), while in the second case (“ETGs”) we combined morphological, photometric, and spectroscopic properties to obtain a more reliable sample of elliptical, red, passive, early-type galaxies (981 galaxies). The analysis is performed at fixed mass to search for any dependence of the color distribution on environment, and at fixed environment to search for any mass dependence. Results. In agreement with the low redshift results of the SDSS, we find that the color distribution of red galaxies is not strongly dependent on environment for all mass bins, exhibiting only a weak trend such that galaxies in overdense regions (log10(1+ δ) ∼ 1.2) are redder than galaxies in underdense regions (log10(1+ δ) ∼ 0.1), with a difference of 〈Δ(U − V)rest〉 = 0.027± 0.008 mag. On the other hand, the dependence on mass is far more significant, and we find that the average colors of massive galaxies (log10(M/M ) ∼ 10.8) are redder by 〈Δ(U − V)rest〉 = 0.093 ± 0.007 mag than low-mass galaxies (log10(M/M ) ∼ 10) throughout the entire redshift range. We study the color-mass (U −V)rest ∝ S M · log10(M/M ) relation, finding a mean slope 〈S M〉 = 0.12± 0.005, while the color-environment (U −V)rest ∝ S δ · log10(1+ δ) relation is flatter, with a slope always smaller than S δ ≈ 0.04. The spectral analysis that we perform on our ETGs sample is in good agreement with our photometric results: we study the 4000 Å break and the equivalent width of the Hδ Balmer line, finding for D4000 a dependence on mass (〈ΔD4000〉 = 0.11 ± 0.02 between log10(M/M ) ∼ 10.2 and log10(M/M ) ∼ 10.8), and a much weaker dependence on environment (〈ΔD4000〉 = 0.05 ± 0.02 between high and low environment quartiles). The same is true for the equivalent width of Hδ, for which we measure a difference of ΔEW0(Hδ) = 0.28 ± 0.08 Å across the same mass range and no significant dependence on environment. By analyzing the lookback time of early-type galaxies, we support the possibility of a downsizing scenario, in which massive galaxies with a stronger D4000 and an almost constant equivalent width of Hδ formed their mass at higher redshift than lower mass ones. We also conclude that the main driver of galaxy evolution is the galaxy mass, the environment playing a subdominant role.