The SCUBA-2 Cosmology Legacy Survey: Multi-wavelength Properties of
  ALMA-identified Submillimeter Galaxies in UKIDSS-UDS
  We present a multi-wavelength analysis of 52 sub-millimeter galaxies (SMGs),
identified using ALMA 870$\mu$m continuum imaging in a pilot program to
precisely locate bright SCUBA2-selected sub-mm sources in the UKIDSS Ultra Deep
Survey (UDS) field. Using the available deep (especially near-infrared),
panoramic imaging of the UDS field at optical-to-radio wavelengths we
characterize key properties of the SMG population. The median photometric
redshift of the bright ALMA/SCUBA-2 UDS (AS2UDS) SMGs that are detected in a
sufficient number of wavebands to derive a robust photometric redshift is
$z$=2.65$\pm$0.13. However, similar to previous studies, 27% of the SMGs are
too faint at optical-to-near-infrared wavelengths to derive a reliable
photometric redshift. Assuming that these SMGs lie at z$\gtrsim$3 raises the
median redshift of the full sample to $z$=2.9$\pm$0.2. A subset of 23,
unlensed, bright AS2UDS SMGs have sizes measured from resolved imaging of their
rest-frame far-infrared emission. We show that the extent and luminosity of the
far-infrared emission are consistent with the dust emission arising from
regions that are optically thick, on average, at a wavelength of
$\lambda_0$$\ge$75$\mu$m (1-$\sigma$ dispersion of 55-90$\mu$m). Using the dust
masses derived from our optically-thick spectral energy distribution models we
determine that these galaxies have a median hydrogen column density of
$N_{H}$=9.8$_{-0.7}^{+1.4}$$\times$10$^{23}$cm$^{-2}$, or a corresponding
median $V$-band obscuration of $A_\mathrm{v}$=540$^{+80}_{-40}$mag, averaged
along the line of sight to the source of their restframe $\sim$200$\mu$m
emission. We discuss the implications of this extreme attenuation by dust for
the multiwavelength study of dusty starbursts and reddening-sensitive tracers
of star formation.
