The zCOSMOS 10k-sample : the role of galaxy stellar mass in the colour-density relation up to z ∼ 1

Aims. With the first similar to 10 000 spectra of the flux limited zCOSMOS sample (I-AB <= 22.5) we want to study the evolution of environmental effects on galaxy properties since z similar to 1.0, and to disentangle the dependence among galaxy colour, stellar mass and local density. Methods. We use our previously derived 3D local density contrast delta, computed with the 5th nearest neighbour approach, to study the evolution with z of the environmental effects on galaxy U-B colour, D4000 angstrom break and [OII]lambda 3727 equivalent width (EW[OII]). We also analyze the implications due to the use of different galaxy selections, using luminosity or stellar mass, and we disentangle the relations among colour, stellar mass and delta studying the colour-density relation in narrow mass bins. Results. We confirm that within a luminosity-limited sample (M-B <= -20.5 -z) the fraction of red (U-B >= 1) galaxies depends on d at least up to z similar to 1, with red galaxies residing mainly in high densities. This trend becomes weaker for increasing redshifts, and it is mirrored by the behaviour of the fraction of galaxies with D4000 angstrom break >= 1.4. We also find that up to z similar to 1 the fraction of galaxies with log(EW[OII]) >= 1.15 is higher for lower delta, and also this dependence weakens for increasing z. Given the triple dependence among galaxy colours, stellar mass and delta, the colour-delta relation that we find in the luminosity-selected sample can be due to the broad range of stellar masses embedded in the sample. Thus, we study the colour-delta relation in narrow mass bins within mass complete subsamples, defining red galaxies with a colour threshold roughly parallel to the red sequence in the colour-mass plane. We find that once mass is fixed the colour-d relation is globally flat up to z similar to 1 for galaxies with log(M/M-circle dot) greater than or similar to 10.7. This means that for these masses any colour-d relation found within a luminosity-selected sample is the result of the combined colour-mass and mass-delta relations. On the contrary, even at fixed mass we observe that within 0.1 <= z <= 0.5 the fraction of red galaxies with log(M/M-circle dot) less than or similar to 10.7 depends on d. For these mass and redshift ranges, environment affects directly also galaxy colours. Conclusions. We suggest a scenario in which the colour depends primarily on stellar mass, but for an intermediate mass regime (10.2 less than or similar to log(M/M-circle dot) less than or similar to 10.7) the local density modulates this dependence. These relatively low mass galaxies formed more recently, in an epoch when more evolved structures were already in place, and their longer SFH allowed environment-driven physical processes to operate during longer periods of time.