The DEEP2 galaxy redshift survey: the evolution of the blue fraction in groups and the field

We explore the behaviour of the blue galaxy fraction over the redshift range 0.75 <= z <= 1.3 in the DEEP2 Survey, both for field galaxies and for galaxies in groups. The primary aim is to determine the role that groups play in driving the evolution of galaxy colour at high z. In pursuing this aim, it is essential to define a galaxy sample that does not suffer from redshift-dependent selection effects in colour-magnitude space. We develop four such samples for this study: at all redshifts considered, each one is complete in colour-magnitude space, and the selection also accounts for evolution in the galaxy luminosity function. These samples will also be useful for future evolutionary studies in DEEP2. The colour segregation observed between local group and field samples is already in place at z similar to 1: DEEP2 groups have a significantly lower blue fraction than the field. At fixed z, there is also a correlation between blue fraction and galaxy magnitude, such that brighter galaxies are more likely to be red, both in groups and in the field. In addition, there is a negative correlation between blue fraction and group richness. In terms of evolution, the blue fraction in groups and the field remains roughly constant from z = 0.75 to 1, but beyond this redshift the blue fraction in groups rises rapidly with z, and the group and field blue fractions become indistinguishable at z similar to 1.3. Careful tests indicate that this effect does not arise from known systematic or selection effects. To further ensure the robustness of this result, we build on previous mock DEEP2 catalogues to develop mock catalogues that reproduce the colour-overdensity relation observed in DEEP2 and use these to test our methods. The convergence between the group and field blue fractions at z similar to 1.3 implies that DEEP2 galaxy groups only became efficient at quenching star formation at z similar to 2; this result is broadly consistent with other recent observations and with current models of galaxy evolution and hierarchical structure growth.