The Hawaii SCUBA-2 Lensing Cluster Survey: Are Low-luminosity
  Submillimeter Galaxies Detected in the Rest-frame UV?
  In this third paper of the Hawaii SCUBA-2 Lensing Cluster Survey series, we
present Submillimeter Array (SMA) detections of six intrinsically faint 850
$\mu$m 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 $\mu$m flux densities of these submillimeter galaxies (SMGs) are
$\sim$ 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 at very high
redshifts. 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 ALMA observations of a large
sample of SCUBA-2 sources in cluster fields will allow us to decide whether the
bimodality we observe here really exists.
