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 \farcs 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 \sim 3 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 } \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 .