Characterizing elusive, faint dusty star-forming galaxies: a lensed,
  optically undetected ALMA galaxy at z~3.3
  We present the serendipitous ALMA detection of a faint submillimeter galaxy
(SMG) lensed by a foreground z~1 galaxy. By optimizing the source detection to
deblend the system, we accurately build the full spectral energy distribution
of the distant galaxy from the I814 band to radio wavelengths. It is extremely
red, with a I-K colour larger than 2.5. We estimate a photometric redshift of
3.28 and determine the physical parameters. The distant galaxy turns out to be
magnified by the foreground lens by a factor of ~1.5, which implies an
intrinsic Ks-band magnitude of ~24.5, a submillimeter flux at 870um of ~2.5 mJy
and a SFR of ~150-300Msun/yr, depending on the adopted tracer. These values
place our source towards the faint end of the distribution of observed SMGs,
and in particular among the still few faint SMGs with a fully characterized
spectral energy distribution, which allows us not only to accurately estimate
its redshift but also to measure its stellar mass and other physical
properties. The galaxy studied in this work is a representative of the
population of faint SMGs, of which only few objects are known to date, that are
undetected in optical and therefore are not typically accounted for when
measuring the cosmic star formation history (SFH). This faint galaxy population
thus likely represents an important and missing piece in our understanding of
the cosmic SFH. Its observation and characterization is of major importance to
achieve a solid picture of galaxy evolution.
