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ABSTRACT Submillimetre galaxies represent a rapid growth phase of both star formation and massive galaxies. Mapping SMGs in galaxy protoclusters provides key insights into where and how these extreme starbursts take place in connections with the assembly of the large-scale structure in the early Universe. We search for SMGs at 850 $\rm{\mu m}$ using JCMT/SCUBA-2 in two massive protoclusters at z = 2.24, BOSS1244 and BOSS1542, and detect 43 and 54 sources with S850 > 4 mJy at the 4σ level within an effective area of 264 arcmin2, respectively. We construct the intrinsic number counts and find that the abundance of SMGs is 2.0 ± 0.3 and 2.1 ± 0.2 times that of the general fields, confirming that BOSS1244 and BOSS1542 contain a higher fraction of dusty galaxies with strongly enhanced star formation. The volume densities of the SMGs are estimated to be ∼15–30 times the average, significantly higher than the overdensity factor (∼6) traced by H α emission-line galaxies (HAEs). More importantly, we discover a prominent offset between the spatial distributions of the two populations in these two protoclusters – SMGs are mostly located around the high-density regions of HAEs, and few are seen inside these regions. This finding may have revealed for the first time the occurrence of violent star formation enhancement in the outskirts of the HAE density peaks, likely driven by the boosting of gas supplies and/or starburst triggering events. Meanwhile, the lack of SMGs inside the most overdense regions at z ∼ 2 implies a transition to the environment disfavouring extreme starbursts. 

ABSTRACT Massive galaxy overdensities at the peak epoch of cosmic star formation provide ideal testbeds for the formation theories of galaxies and large-scale structure. We report the confirmation of two massive galaxy overdensities at z = 2.24, BOSS1244 and BOSS1542, selected from the Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH) project using Lyα absorption from the intergalactic medium over the scales of 15−30 h−1 Mpc imprinted on the quasar spectra. We use Hα emitters (HAEs) as the density tracer and identify them using deep narrow-band H2S(1) and broad-band Ks imaging data obtained with the wide-field infrared camera (WIRCam) at the Canada–France–Hawaii Telescope. In total, 244 and 223 line emitters are detected in these two fields, and 196 ± 2 and 175 ± 2 are expected to be HAEs with an Hα flux of >2.5 × 10−17 erg s−1 cm−2 (corresponding to a star formation rate of >5 M⊙ yr−1). The detection rate of HAE candidates suggests an overdensity factor of δgal = 5.6 ± 0.3 and 4.9 ± 0.3 over the volume of 54 × 32 × 32 co-moving Mpc3. The overdensity factor increases two to three times when focusing on the high-density regions of scales 10–15  co-moving Mpc. Interestingly, the HAE density maps reveal that BOSS1244 contains a dominant structure, while BOSS1542 manifests as a giant filamentary structure. We measure the Hα luminosity functions (HLFs), finding that BOSS1244’s HLF is nearly identical to that of the general field at the same epoch, while BOSS1542 shows an excess of HAEs with high Hα luminosity, indicating the presence of enhanced star formation or active galactic nuclei activity. We conclude that the two massive MAMMOTH overdensities are undergoing a rapid galaxy mass assembly.