A deficit of faint red galaxies in the possible large-scale structures around the RDCS J1252.9-2927 cluster at z=1.24

We report a discovery of possible large-scale structures around the RDCS J1252.9-2927 cluster at z = 1.24 based on photometric redshifts. We carried out multiband wide-field imaging with Suprime-Cam on the Subaru Telescope and WFCAM on the United Kingdom Infrared Telescope (UKIRT). Our data cover an approximately 35 x 30-arcmin(2) field in V, R, i', z' and K bands. We apply a photometric redshift technique to extract galaxies at or near the cluster redshift. The distribution of photo-z selected galaxies reveals clumpy structures surrounding the central cluster. It seems that there is a large (> 20 Mpc) filamentary structure extending in the north-east -south-west direction. We compare the observed structure with an X-ray map and find that two of the four plausible clumps show significant X-ray emissions and one with a marginal detection, which strongly suggests that they are dynamically bound systems. Following the discovery of the possible large-scale structure, we carried out deeper SOFI K-s-band imaging with the New Technology Telescope on four plausible clumps. We construct the optical-to-near-infrared colour-magnitude diagrams of the galaxies in the clumps, and find that the colour-magnitude relation (CMR) of the red galaxies in the clumps is sharply truncated below K-s = 22. Few faint red galaxies are seen in these clumps. This suggests that the CMR first appears at the bright magnitudes and it extends to the fainter magnitudes with time, which is consistent with the `downsizing' picture. Interestingly, the CMR of the main cluster has previously been shown to have a clear relation down to K-s = 23. This confirms our previous suggestion that the build-up of the CMR is delayed in low-density environments. All in all, we suggest that galaxies follow the `environment-dependent downsizing' evolution. Massive galaxies in high-density environments first stop forming stars and become red. Less massive galaxies in less dense environments become red at later times. Based on a few assumptions, we predict that the brightest tip of the CMR appears at z similar to 2.5