We present SCUBA-2 and SMA follow-up observations of four candidate high redshift Dusty Star-Forming Galaxies, selected as sources with rising SEDs in the 250, 350 and 500 ๐m Herschel SPIRE bands. Previous SMA observations showed no counterparts to these sources, but in our deeper sub-mm observations we detect counterparts to all four 500 ๐m risers, with three resolving into multiple systems. For these three multiple systems, the SMA 345 GHz (โ 870 ๐m) observations recover 123 ยฑ 73%, 60 ยฑ 15% and 19 ยฑ 4% respectively of the integrated 850 ๐m flux density from SCUBA-2, indicating that there may be additional sources below our SMA detection limit making up a dense, protocluster core. The fourth 500 ๐m riser was observed at a lower frequency and so we cannot make a similar comparison. We estimate photometric redshifts based on FIR/sub-mm colours, finding that 3/4 likely lie at ๐ง โฅ 2. This fits with the interpretation that the 500 ๐m riser selection criterion selects both intrinsically red, individual galaxies at ๐ง > 4, and multiple systems at more moderate redshifts, artificially reddened by the effects of blending. We use the SCUBA-2 850 ๐m maps to investigate the environments of these 500 ๐m risers. By constructing cumulative number counts and estimating photometric redshifts for surrounding SCUBA-2 detections, we find that one of our 500 ๐m risers could plausibly reside in a ๐ง โฅ 2 protocluster. We infer that bright 500 ๐m risers with faint 850 ๐m flux densities are typically multiple systems at ๐ง โฅ 2 that may reside in overdensities of bright sub-mm galaxies.
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