2017ApJ...850..189H

In this third paper of the Hawaii SCUBA-2 Lensing Cluster Survey series, we present Submillimeter Array (SMA) detections of six intrinsically faint 850 µm sources detected in SCUBA-2 images of the lensing cluster fields, A1689, A2390, A370, MACS J0717.5+3745, and MACS J1423.8+2404. Two of the SCUBA-2 sources split into doublets, yielding a total of eight SMA detections. The intrinsic 870 µm flux densities of these submillimeter galaxies (SMGs) are ∼1 mJy. Five of the eight SMGs are not detected in optical or near-infrared (NIR) images. The NIR-to-submillimeter flux ratios of these faint SMGs suggest that most of them are extremely dusty and/or are at very high redshifts. By combining these SMGs and several other samples from the literature, we find a bimodal distribution for the faint sources in the space of submillimeter flux versus NIR-to-submillimeter flux ratio. While most of the SMA-detected lensed sources are very obscured, the other SMGs with similar flux densities are mostly bright in the NIR. Future Atacama Large Millimeter/submillimeter Array observations of a large sample of SCUBA-2 sources in cluster fields will allow us to decide whether or not the bimodality we observe here really exists.