2020A&A...640L...8U

Deep surveys with the Atacama Large Millimeter Array (ALMA) have uncovered a population of dusty star-forming galaxies which are faint or even undetected at optical to near-infrared wavelengths. Their faintness at short wavelengths makes the detailed characterization of the population challenging. Here we present a spectroscopic redshift identification and a characterization of one of these near-infrared-dark galaxies discovered by an ALMA deep survey. The detection of [CI](1-0) and CO(4-3) emission lines determines the precise redshift of the galaxy, ADF22.A2, to be z=3.9913±0.0008. On the basis of a multi-wavelength analysis, ADF22.A2 is found to be a massive, star-forming galaxy with a stellar mass of M*=1.1_–0.6^+1.3x10¹¹M_☉ and SFR=430_–150⁺²³⁰M_☉/yr. The molecular gas mass was derived to be M(H₂)^[CI]=(5.9±1.5)x10¹⁰M_☉, indicating a gas fraction of ≃35%, and the ratios of L_[CI](1–0)/L_IR and L_[CI](1–0)/L_CO(4–3) suggest that the nature of the interstellar medium in ADF22.A2 is in accordance with those of other bright submillimeter galaxies. The properties of ADF22.A2, including the redshift, star-formation rate, stellar mass, and depletion time scale (τ_dep≃0.1-0.2Gyr), also suggest that ADF22.A2 has the characteristics expected for the progenitors of quiescent galaxies at z≥3. Our results demonstrate the power of ALMA contiguous mapping and line scan, which help us to obtain an unbiased view of galaxy formation in the early Universe.