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We carried out deep H$\alpha$ narrowband imaging with 10 hours netintegrations towards the young protocluster, USS1558$-$003 at $z=2.53$ with theSubaru Telescope. This system is composed of four dense groups with massivelocal overdensities, traced by 107 H$\alpha$ emitters (HAEs) with stellarmasses and dust-corrected star formation rates down to $1\times10^8$ M$_\odot$and 3 M$_\odot$yr$^{-1}$, respectively. We have investigated the environmentaldependence of various physical properties within the protocluster by comparingdistributions of HAEs in higher and lower densities with a standardKolmogorov--Smirnov test. At 97\% confidence level, we find enhanced starformation across the star-forming main sequence of HAEs living in the mostextreme `supergroup', corresponding to the top quartile of overdensities.Furthermore, we derive distribution functions of H$\alpha$ luminosity andstellar mass in group and intergroup regions, approximately corresponding to 30times and 8 times higher densities than the general field. As a consequence, weidentify by 0.7 and 0.9 dex higher cutoffs in H$\alpha$ luminosity and stellarmass functions in the dense groups, respectively. On the other hand, HAEs inthe intergroup environment of the protocluster show similar distributionfunctions to those of field galaxies despite residing in significantoverdensities. In the early phase of cluster formation, as inferred from ourresults, the densest parts in the protocluster have had an acceleratedformation of massive galaxies. We expect that these eventually grow andtransform into early-type galaxies at the bright end of the red sequence asseen in present-day rich clusters of galaxies.

MAHALO Deep Cluster Survey I. Accelerated and enhanced galaxy formation in the densest regions of a protocluster at z=2.5