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 ( log _ { 10 } ( 1 + \delta ) \sim 1.2 ) are redder than galaxies in underdense regions ( log _ { 10 } ( 1 + \delta ) \sim 0.1 ) , with a difference of \left < \Delta ( U - V ) _ { rest } \right > = 0.027 \pm 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 ( log _ { 10 } ( M / M _ { \odot } ) \sim 10.8 ) are redder by \left < \Delta ( U - V ) _ { rest } \right > = 0.093 \pm 0.007 mag than low-mass galaxies ( log _ { 10 } ( M / M _ { \odot } ) \sim 10 ) throughout the entire redshift range .
We study the color-mass ( U - V ) _ { rest } \propto S _ { M } \cdot log _ { 10 } ( M / M _ { \odot } ) relation , finding a mean slope \left < S _ { M } \right > = 0.12 \pm 0.005 , while the color-environment ( U - V ) _ { rest } \propto S _ { \delta } \cdot log _ { 10 } ( 1 + \delta ) relation is flatter , with a slope always smaller than S _ { \delta } \approx 0.04 . The spectral analysis that we perform on our ETGs sample is in good agreement with our photometric results : we study the 4000 Åbreak and the equivalent width of the H \delta Balmer line , finding for D4000 a dependence on mass ( \left < \Delta D 4000 \right > = 0.11 \pm 0.02 between log _ { 10 } ( M / M _ { \odot } ) \sim 10.2 and log _ { 10 } ( M / M _ { \odot } ) \sim 10 .8 ) , and a much weaker dependence on environment ( \left < \Delta D 4000 \right > = 0.05 \pm 0.02 between high and low environment quartiles ) .
The same is true for the equivalent width of H \delta , for which we measure a difference of \Delta EW _ { 0 } ( H \delta ) = 0.28 \pm 0.08 Åacross 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 H \delta 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 .