Multi-band Optical and Near-infrared Properties of Faint Submillimeter
  Galaxies with Serendipitous ALMA Detections
  We present a catalog of 26 faint submillimeter galaxies (SMGs) in the XMM-LSS
field identified by cross-matching serendipitously detected sources in archival
ALMA Band 6 and 7 data with multi-band near-infrared (NIR) and optical data
from the Spitzer Extragalactic Representative Volume Survey, the VISTA Deep
Extragalactic Survey, the Canada-France-Hawaii Telescope Legacy Large Survey,
and the Hyper Suprime-Cam Subaru Strategic Program. Of the 26 SMGs in our
sample, 15 are identified here for the first time. The majority of the sources
in our sample (16/26) have faint submm fluxes ($0.1\,{\rm mJy} < S_{\rm 1\,mm}
< 1\,{\rm mJy}$). In addition to the 26 SMGs with multi-band optical and NIR
detections, there are 60 highly-reliable ($>5\sigma$) ALMA sources with no
counterpart in any other band down to an IRAC [4.5] $AB$ magnitude of $\approx
23.7$. To further characterize the 26 galaxies with both ALMA and optical/NIR
counterparts, we provide 13-band forced photometry for the entire catalog using
the Tractor and calculate photometric redshifts and rest-frame colors. The
median redshift of our sample is $\langle z \rangle = 2.66$. We find that our
sample galaxies have bluer colors compared to bright SMGs, and the UVJ color
plot indicates that their colors are consistent with main sequence star-forming
galaxies. Our results provide new insights into the nature of the faint
population of SMGs, and also highlight opportunities for galaxy evolution
studies based on archival ALMA data.
