Discovery of a dark, massive, ALMA-only galaxy at z~5-6 in a tiny
  3-millimeter survey
  We report the serendipitous detection of two 3 mm continuum sources found in
deep ALMA Band 3 observations to study intermediate redshift galaxies in the
COSMOS field. One is near a foreground galaxy at 1.3", but is a previously
unknown dust-obscured star-forming galaxy (DSFG) at probable $z_{CO}=3.329$,
illustrating the risk of misidentifying shorter wavelength counterparts. The
optical-to-mm spectral energy distribution (SED) favors a grey $\lambda^{-0.4}$
attenuation curve and results in significantly larger stellar mass and SFR
compared to a Calzetti starburst law, suggesting caution when relating
progenitors and descendants based on these quantities. The other source is
missing from all previous optical/near-infrared/sub-mm/radio catalogs
("ALMA-only"), and remains undetected even in stacked ultradeep optical
($>29.6$ AB) and near-infrared ($>27.9$ AB) images. Using the ALMA position as
a prior reveals faint $SNR\sim3$ measurements in stacked IRAC 3.6+4.5,
ultradeep SCUBA2 850$\mu$m, and VLA 3GHz, indicating the source is real. The
SED is robustly reproduced by a massive $M^*=10^{10.8}$M$_\odot$ and
$M_{gas}=10^{11}$M$_\odot$, highly obscured $A_V\sim4$, star forming
$SFR\sim300$ M$_{\odot}$yr$^{-1}$ galaxy at redshift $z=5.5\pm$1.1. The
ultrasmall 8 arcmin$^{2}$ survey area implies a large yet uncertain
contribution to the cosmic star formation rate density CSFRD(z=5)
$\sim0.9\times10^{-2}$ M$_{\odot}$ yr$^{-1}$ Mpc$^{-3}$, comparable to all
ultraviolet-selected galaxies combined. These results indicate the existence of
a prominent population of DSFGs at $z>4$, below the typical detection limit of
bright galaxies found in single-dish sub-mm surveys, but with larger space
densities $\sim3 \times 10^{-5}$ Mpc$^{-3}$, higher duty cycles $50-100\%$,
contributing more to the CSFRD, and potentially dominating the high-mass galaxy
stellar mass function.
