The Nature of 500 Micron Risers II: Sub-mm Faint Dusty Star Forming Galaxies

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$\mu$m 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$\mu$m risers, with three resolving into multiple systems. For these three multiple systems, the SMA 345GHz ($\approx 870\mu$m)observationsrecover$123 \pm 73%$,$60 \pm 15%$and$19 \pm 4%$respectivelyoftheintegrated850$\mu$m 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$\mu$m 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 $z \geq 2$\mathrm{.}Thisfitswiththeinterpretationthatthe500$\mu$m riser selection criterion selects both intrinsically red, individual galaxies at $z>4$, and multiple systems at more moderate redshifts, artificially reddened by the effects of blending. We use the SCUBA-2 850$\mu$m maps to investigate the environments of these 500$\mu$m risers. By constructing cumulative number counts and estimating photometric redshifts for surrounding SCUBA-2 detections, we find that one of our 500$\mu$m risers could plausibly reside in a $z\ge 2$ protocluster. We infer that bright 500$\mu$m risers with faint 850$\mu$m flux densities are typically multiple systems at $z\ge 2$ that may reside in overdensities of bright sub-mm galaxies.