The Role of Galaxy Stellar Mass in the Colour–Density Relation up to z ~ 1

It is well known that galaxy properties correlate with the local environment in which galaxies reside. In contrast, it is still matter of debate why and when these environmental dependences originate, and whether only one “main” property depends on environment, thus driving all the others environmental dependences via the correlations among properties themselves. We use the first ∼ 10, 000 spectra of the zCOSMOS sample (I ≤ 22. 5) to study the role of galaxy stellar mass in the colour–density relation up to z = 1. We confirm that within a luminosity-limited sample (M B ≤ − 20. 5 − z) red galaxies reside mainly in high densities (δ) at least up to z = 1. This trend becomes weaker for increasing redshifts, and it is mirrored by the D n 4, 000 \r{A} break–density relation. We also find that up to z ∼ 1 the fraction of galaxies with log(EW[OII]) ≥ 1. 15 is higher for lower δ. Given the triple dependence among galaxy colours, stellar mass and δ, the colour–δ relation that we find can be due to the broad range of stellar masses embedded in the sample. We find that once mass is fixed the colour-δ relation is globally flat up to z ∼ 1 for galaxies with log(M ∕ M sun ) ≥ 10. 7. 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 sun ) ≤ 10. 7 depends on δ. We suggest a scenario in which the colour depends primarily on stellar mass, but for an intermediate mass regime (10. 2 ≤ log(M ∕ M sun ) ≤ 10. 7) the local density modulates this dependence. These lower mass 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.