We present environmental dependence of the build-up of the colour-magnitude relation (CMR) at z ~ 0.8. It is well established that massive early-type galaxies exhibit a tight CMR in clusters up to at least z ~ 1. The faint end of the relation, however, has been much less explored especially at high redshifts primarily due to limited depths of the data. Some recent papers have reported a deficit of the faint red galaxies on the CMR at 0.8 < z < 1, but this has not been well confirmed yet and is still controversial. Using a deep, multi-colour, panoramic imaging data set of the distant cluster RXJ1716.4+6708 at z=0.81, newly taken with the Prime Focus Camera (Suprime-Cam) on the Subaru Telescope, we carry out an analysis of faint red galaxies with a care for incompleteness. We find that there is a sharp decline in the number of red galaxies toward the faint end of the CMR below M*+2. We compare our result with those for other clusters at z ~ 0.8 taken from the literature, which show or do not show the deficit. We suggest that the deficit of faint red galaxies is dependent on the richness or mass of the clusters, in the sense that poorer systems show stronger deficits. This indicates that the evolutionary stage of less massive galaxies depends critically on environment.
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