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We report the serendipitous detection of two 3 mm continuum sources found indeep ALMA Band 3 observations to study intermediate redshift galaxies in theCOSMOS field. One is near a foreground galaxy at 1.3", but is a previouslyunknown dust-obscured star-forming galaxy (DSFG) at probable $z_{CO}=3.329$,illustrating the risk of misidentifying shorter wavelength counterparts. Theoptical-to-mm spectral energy distribution (SED) favors a grey $\lambda^{-0.4}$attenuation curve and results in significantly larger stellar mass and SFRcompared to a Calzetti starburst law, suggesting caution when relatingprogenitors and descendants based on these quantities. The other source ismissing 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 asa 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. TheSED 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$1.1. Theultrasmall 8 arcmin$^{2}$ survey area implies a large yet uncertaincontribution to the cosmic star formation rate density CSFRD(z=5)$\sim0.9\times10^{-2}$ M$_{\odot}$ yr$^{-1}$ Mpc$^{-3}$, comparable to allultraviolet-selected galaxies combined. These results indicate the existence ofa prominent population of DSFGs at $z>4$, below the typical detection limit ofbright galaxies found in single-dish sub-mm surveys, but with larger spacedensities $\sim3 \times 10^{-5}$ Mpc$^{-3}$, higher duty cycles $50-100\%$,contributing more to the CSFRD, and potentially dominating the high-mass galaxystellar mass function.

Discovery of a dark, massive, ALMA-only galaxy at z~5-6 in a tiny 3-millimeter survey