The SCUBA-2 Cosmology Legacy Survey: Multi-wavelengths counterparts to
  10$^3$ submillimeter galaxies in the UKIDSS-UDS field
  We present multiwavelength identifications for the counterparts of 1088
submillimeter sources detected at 850$\mu$m in the SCUBA-2 Cosmology Legacy
Survey study of the UKIDSS-UDS field. By utilising an ALMA pilot study on a
subset of our bright SCUBA-2 sample as a training set, along with the deep
optical-near-infrared data available in this field, we develop a novel
technique, Optical-IR Triple Color (OIRTC), using $z-K$, $K-[3.6]$,
$[3.6]-[4.5]$ colors to select the candidate submillimeter galaxy (SMG)
counterparts. By combining radio identification and the OIRTC technique, we
find counterpart candidates for 80% of the Class = 1 $\geq4\,\sigma$ SCUBA-2
sample, defined as those that are covered by both radio and OIR imaging and the
base sample for our scientific analyses. Based on the ALMA training set, we
expect the accuracy of these identifications to be $82\pm20$%, with a
completeness of $69\pm16$%, essentially as accurate as the traditional
$p$-value technique but with higher completeness. We find that the fraction of
SCUBA-2 sources having candidate counterparts is lower for fainter 850$\mu$m
sources, and we argue that for follow-up observations sensitive to SMGs with
$S_{850}\gtrsim 1$ mJy across the whole ALMA beam, the fraction with multiple
counterparts is likely to be $>40$% for SCUBA-2 sources at $S_{850} \gtrsim 4$
mJy. We find that the photometric redshift distribution for the SMGs is well
fit by a lognormal distribution, with a median redshift of $z=2.3\pm0.1$. After
accounting for the sources without any radio and/or OIRTC counterpart, we
estimate the median redshift to be $z=2.6\pm0.1$ for SMGs with $S_{850} >1$
mJy. We also use this new large sample to study the clustering of SMGs and the
the far-infrared properties of the unidentified submillimeter sources by
stacking their Herschel SPIRE far-infrared emission.
