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 [ C i ] ( 1-0 ) and CO ( 4-3 ) emission lines determines the precise redshift of the galaxy , ADF22.A2 , to be z = 3.9913 \pm 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 } ^ { +1.3 } \times 10 ^ { 11 } M _ { \odot } and SFR = 430 _ { -150 } ^ { +230 } M _ { \odot } yr ^ { -1 } .
The molecular gas mass was derived to be M ( H _ { 2 } ) ^ { [ CI ] } = ( 5.9 \pm 1.5 ) \times 10 ^ { 10 } M _ { \odot } , indicating a gas fraction of \approx 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 ( \tau _ { dep } \approx 0.1 - 0.2 Gyr ) , also suggest that ADF22.A2 has the characteristics expected for the progenitors of quiescent galaxies at z \gtrsim 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 .