2019ApJ...878...73Y

We report detections of two 1.2 mm continuum sources (S_1.2 mm_ ∼ 0.6 mJy) without any counterparts in the deep H- and/or K-band image (i.e., K-band magnitude >=26 mag). These near-infrared-dark faint millimeter sources are uncovered by ASAGAO, a deep and wide-field (≃26 arcmin²) Atacama Large Millimeter/submillimeter Array (ALMA) 1.2 mm survey. One has a red IRAC (3.6 and 4.5 µm) counterpart, and the other has been independently detected at 850 and 870 µm using SCUBA2 and ALMA Band 7, respectively. Their optical-to-radio spectral energy distributions indicate that they can lie at z >= 3-5 and can be in the early phase of massive galaxy formation. Their contribution to the cosmic star formation rate density is estimated to be ∼1 x 10^–3 M_☉ yr^–1 Mpc^–3 if they lie somewhere in the redshift range of z ∼ 3-5. This value can be consistent with, or greater than, that of bright submillimeter galaxies (S_870 µm_ > 4.2 mJy) at z ∼ 3-5. We also uncover three more candidate near-infrared-dark faint ALMA sources without any counterparts (S_1.2 mm_ ∼ 0.45-0.86 mJy). These results show that an unbiased ALMA survey can reveal the dust-obscured star formation activities, which were missed in previous deep optical/near-infrared surveys.