The DEEP2 Galaxy Redshift Survey: the relationship between galaxy properties and environment at z ~ 1

We study the mean environment of galaxies in the DEEP2 Galaxy Redshift Survey as a function of rest-frame colour, luminosity, and [OII] 3727Å equivalent width. The local galaxy overdensity for >14000 galaxies at 0.75 < z < 1.35 is estimated using the projected third-nearest-neighbour surface density. Of the galaxy properties studied, mean environment is found to depend most strongly on galaxy colour; all major features of the correlation between mean overdensity and rest-frame colour observed in the local universe were already in place at z ~ 1. In contrast to local results, we find a substantial slope in the mean dependence of environment on luminosity for blue, star forming galaxies at z ~ 1, with brighter blue galaxies being found on average in regions of greater overdensity. We discuss the roles of galaxy clusters and groups in establishing the observed correlations between environment and galaxy properties at high redshift, and we also explore the evidence for a `downsizing of quenching' from z ~ 1 to ~0. Our results add weight to existing evidence that the mechanism(s) that result in star formation quenching are efficient in group environments as well as clusters. This work is the first of its kind at high redshift and represents the first in a series of papers addressing the role of environment in galaxy formation at 0 < z < 1.