ALMA Deep Field in SSA22: A near-infrared-dark submillimeter galaxy at
  z=4.0
  Deep surveys with 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 detailed characterization of the population challenging. Here we present
a spectroscopic redshift identification and characterization of one of such
near-infrared-dark galaxy discovered by an ALMA deep survey. 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 multi-wavelength
analysis, ADF22.A2 is found to be a massive, star-forming galaxy with stellar
mass Mstar = $1.1_{-0.6}^{+1.3}$ x 10^{11} Msun and SFR = $430_{-150}^{+230}$
Msun/yr. The molecular gas mass is derived to be M ($H_2$) = 5.9 +/-
1.5x10^{10} Msun, indicating a gas fraction of ~35%, and the ratios of $L_{\rm
[CI](1-0)}/L_{\rm IR}$ and $L_{\rm [CI](1-0)}/L_{\rm CO(4-3)}$ suggests 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
redshift, star-formation rate, stellar mass, and depletion time scale
(tau~0.1-0.2 Gyr), 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 to obtain an unbiased view of
galaxy formation in the early Universe.
