Absence of evidence is not evidence of absence: The colour-density relation at fixed stellar mass persists to z 1

We use data drawn from the DEEP2 Galaxy Redshift Survey to investigate the relationship between local galaxy density, stellar mass and rest-frame galaxy colour. At z 0.9, we find that the shape of the stellar mass function at the high-mass [ log ₁₀(M_*/h^-2 M ) > 10.1] end depends on the local environment, with high-density regions favouring more massive systems. Accounting for this stellar mass-environment relation (i.e. working at fixed stellar mass), we find a significant colour-density relation for galaxies with 10.6 < log ₁₀(M_*/h^-2 M ) < 11.1 and 0.75 < z < 0.95. This result is shown to be robust to variations in the sample selection and to extend to even lower masses [down to log ₁₀(M_*/h^-2 M ) 10.4]. We conclude by discussing our results in comparison to recent works in the literature, which report no significant correlation between galaxy properties and environment at fixed stellar mass for the same redshift and stellar mass domain. The non-detection of environmental dependence found in other data sets is largely attributable to their smaller sample sizes and lower sampling density, as well as systematic effects such as inaccurate redshifts and biased analysis techniques. Ultimately, our results based on DEEP2 data illustrate that the evolutionary state of a galaxy at z 1 is not exclusively determined by the stellar mass of the galaxy. Instead, we show that local environment appears to play a distinct role in the transformation of galaxy properties at z > 1.