An enhanced abundance of bright galaxies in protocluster candidates at z 3-5

We present a protocluster search covering z ~ 3 to z ~ 5 based on the combination of the Hyper SuprimeCam Subaru Strategic Programme and the CFHT Large Area U-band Deep Survey. We identify about 30 protocluster candidates per unit redshift over the $\sim 25\, \mathrm{deg^2}$ area of the Deep/UltraDeep layer. Protocluster candidates are selected as regions with a significantly enhanced surface density of dropout galaxies. With this large sample, we characterize the properties of their individual member galaxies. We compare the number counts of dropout galaxies in protocluster candidates with that of coeval field galaxies. Rest-frame ultraviolet (UV) bright galaxies are overabundant in protocluster candidates, a trend seen across the full redshift range studied. We do not find evidence for their spatial distribution within protocluster candidates to be distinct from their fainter counterparts, nor for their UV colour to be different from that of field galaxies with the same brightness. Cosmological simulations predict this bright-end excess, with the main cause being a richer population of massive galaxies, with only a minor contribution from an enhancement in star formation activity (and therefore UV emission) at fixed mass. U-to-K SED modelling of our observed samples supports this interpretation. This environmental differentiation in number counts is already in place at z ~ 5, with no significant redshift dependence over the range in lookback times probed . These observational results and model predictions suggest that the cosmic clock is ahead in high-density environments.