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

TLDR

The study investigates how galaxy evolution depends on environment using deep imaging of two distant clusters from the PISCES project. They built a large galaxy sample by combining deep Suprime‑Cam imaging of two distant clusters with SDSS data, then classified environments into field, groups, and clusters using local and global galaxy densities. The results show that faint galaxies shift from blue to red at a critical density, the cluster colour–magnitude relation is already established by z = 0.83, while the bright end of the field relation continues to build to the present and the faint end remains undeveloped, indicating a down‑sizing evolution that is delayed in lower‑density environments.

Abstract

We discuss the environmental dependence of galaxy evolution based on deep panoramic imaging of two distant clusters taken with Suprime-Cam as part of the PISCES project. By combining with the SDSS 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). 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. Based on local and global densities of galaxies, we classify three environments: field, groups, and clusters. We show that the cluster colour-magnitude relation is already built at $z=0.83$. 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 the '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.