The build-up of the colour-magnitude relation as a function of environment

Abstract

We discuss the environmental dependence of galaxy evolution based on deep panoramic imaging of two distant clusters, RX J0152.7-1357 at z = 0.83 and CL0016+ 1609 at z = 0.55, taken with the Subaru Prime Focus Camera on the Subaru Telescope as part of the Panoramic Imaging and Spectroscopy of Cluster Evolution with Subaru project. By combining with the Sloan Digital Sky Survey data as a local counterpart for comparison, we construct a large sample of galaxies that spans wide ranges in environment, time and stellar mass (or luminosity). This allows us to conduct systematic and statistical analyses of the photometric properties of galaxies based on the colour-density diagrams, colour-magnitude relations, and luminosity functions. We find that colours of galaxies, especially those of faint galaxies (M V > M v * + 1), change from blue to red at a break density as we go to denser regions. This trend is observed at all redshifts in our sample. Based on local and global densities of galaxies, we classify three environments - field, groups and clusters - and look into the environmental dependence of galaxies in detail. In particular, we quantify how the colour-magnitude relation is built up as a function of environment. We show that the bright end of the cluster colour-magnitude relation is already built a z = 0.83, while the faint end is possibly still in the process of build-up. In contrast to this, the bright end of the field colour-magnitude relation has been vigorously built all the way down to the present-day and the build-up at the faint end has not started yet. A possible interpretation of these results is that galaxies evolve in a 'down-sizing' fashion. That is, massive galaxies complete their star formation first and the truncation of star formation is propagated to smaller objects as time progresses. This trend is likely to depend on environment since the build-up of the colour-magnitude relation is delayed in lower density environments. Therefore, we may suggest that the evolution of galaxies took place earliest in massive galaxies and in high-density regions, and it is delayed in less massive galaxies and in lower density regions. Further studies are, however, obviously needed to confirm the observed trends and establish the 'down-sizing' picture.