(Abridged) 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 multi-band wide-field imaging with Suprime-Cam on the Subaru
Telescope and WFCAM on the United Kingdom Infra-Red Telescope (UKIRT). The
distribution of photo-z selected galaxies reveals clumpy structures surrounding
the central cluster. 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 suggest 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 New Technology Telescope on
the 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. Interestingly, the main cluster shows a clear relation
down to K_s=23 (Lidman et al. 2004). We suggest that galaxies follow the
'environment-dependent down-sizing' 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~2.5.Comment: 10 pages, 8 figures, accepted for publication in MNRA