The zCOSMOS 10k-sample: the role of galaxy stellar mass in the
  colour-density relation up to z=1
  [Abridged] With the first 10000 spectra of the flux limited zCOSMOS sample
(I<=22.5) we study the evolution of environmental effects on galaxy properties
since z=1.0, and disentangle the dependence among galaxy colour, stellar mass
and local density (3D local density contrast `delta', computed with the 5th
nearest neighbour approach). We confirm that within a luminosity-limited sample
(M_B<=-20.5-z) the fraction of red (U-B>=1) galaxies 'f_red' depends on delta
at least up to z=1, with red galaxies residing mainly in high densities. This
trend weakens for increasing z, and it is mirrored by the behaviour of the
fraction of galaxies with D4000A break >=1.4. We also find that up to z=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 found
in the luminosity-selected sample can be due to the broad range of stellar
masses. Thus, we fix the stellar mass and we find that in this case the
colour-delta relation is flat up to z=1 for galaxies with log(M/M_sun)>=10.7.
This means that for these masses the colour-delta relation found in a
luminosity-selected sample is the result of the combined colour-mass and
mass-delta relations. In contrast, we find that for 0.1<=z<=0.5 and
log(M/M_sun)<=10.7 'f_red' depends on delta even at fixed mass. In these mass
and z ranges, environment affects directly also galaxy colours. We suggest a
scenario in which the colour depends primarily on stellar mass, but for
relatively low mass galaxies the local density modulates this dependence. These
galaxies formed more recently, in an epoch when evolved structures were already
in place, and their longer SFH allowed environment-driven physical processes to
operate during longer periods of time.
