Discovery of a Dark, Massive, ALMA-only Galaxy at z ~ 5–6 in a Tiny 3 mm 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 13, 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-millimeter spectral energy distribution (SED) favors a gray λ ^−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/submillimeter/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 signal-to-noise ratio ~ 3 measurements in stacked IRAC 3.6+4.5, ultradeep SCUBA2 850 μm, and VLA 3 GHz, indicating the source is real. The SED is robustly reproduced by a massive M* = 10^10.8 M _⊙ and M _gas = 10¹¹ M _⊙, highly obscured A _V  ~ 4, star-forming SFR ~ 300 M _⊙ yr⁻¹ galaxy at redshift z = 5.5 ± 1.1. The ultrasmall 8 arcmin² survey area implies a large yet uncertain contribution to the cosmic star formation rate density CSFRD(z = 5) ~ 0.9 × 10⁻² M _⊙ 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 submillimeter surveys, but with larger space densities ~3 × 10⁻⁵ Mpc⁻³, higher duty cycles of 50%–100%, contributing more to the CSFRD, and potentially dominating the high-mass galaxy stellar mass function.