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

Abstract

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 $\hat{I}{»}^{-0.4}$ 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 $SNR\sim 3$ measurements in stacked IRAC 3.6+4.5, ultradeep SCUBA2 850$\hat{I}¼$m, and VLA 3GHz, indicating the source is real. The SED is robustly reproduced by a massive $M^{\ast }=10^{10.8}$M${}_{\odot }$ and $M_{gas}=10^{11}$M${}_{\odot }$, highly obscured $A_V\sim 4$, star forming $SFR\sim 300$ 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) $\sim 0.9\times 10^{-2}$ 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 $\sim 3\times 10^{-5}$ Mpc${}^{-3}$, higher duty cycles $50-100\%$, contributing more to the CSFRD, and potentially dominating the high-mass galaxy stellar mass function.

Accepted for publication in ApJ. 2 galaxies, too many pages, 8 figures, 2 tables