ALMA Deep Field in SSA22: A near-infrared-dark submillimeter galaxy at z = 4.0

Abstract

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 [Ca I](1-0) and CO(4-3) emission lines determines the precise redshift of the galaxy, ADF22.A2, to be za =a 3.9913a ±a 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 Ma - = 1.1-0.6+1.3 × 1011 MaS and SFR = 430-150+230a MaS yr-1. The molecular gas mass was derived to be M(H2)[CI]a =a (5.9a ±a 1.5)×1010a MaS , indicating a gas fraction of ≈35%, and the ratios of L[CI](1-0)/LIR and L[CI](1-0)/LCO(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 (τdepa ≈a 0.1-0.2 Gyr), also suggest that ADF22.A2 has the characteristics expected for the progenitors of quiescent galaxies at zaaa 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.