Faint Submillimeter Galaxies Revealed by Multifield Deep ALMA
  Observations: Number Counts, Spatial Clustering, and A Dark Submillimeter
  Line Emitter
  We present the statistics of faint submillimeter/millimeter galaxies (SMGs)
and serendipitous detections of a submillimeter/millimeter line emitter (SLE)
with no multi-wavelength continuum counterpart revealed by the deep ALMA
observations. We identify faint SMGs with flux densities of 0.1-1.0 mJy in the
deep Band 6 and Band 7 maps of 10 independent fields that reduce cosmic
variance effects. The differential number counts at 1.2 mm are found to
increase with decreasing flux density down to 0.1 mJy. Our number counts
indicate that the faint (0.1-1.0 mJy, or SFR_IR ~ 30-300 Msun/yr) SMGs
contribute nearly a half of the extragalactic background light (EBL), while the
remaining half of the EBL is mostly contributed by very faint sources with flux
densities of <0.1 mJy (SFR_IR <~ 30 Msun/yr). We conduct counts-in-cells
analysis with the multifield ALMA data for the faint SMGs, and obtain a coarse
estimate of galaxy bias, b_g <4. The galaxy bias suggests that the dark halo
masses of the faint SMGs are <~ 7x10^12 Msun, which is smaller than those of
bright (>1 mJy) SMGs, but consistent with abundant high-z star-forming
populations such as sBzKs, LBGs, and LAEs. Finally, we report the serendipitous
detection of SLE-1 with continuum counterparts neither in our 1.2 mm-band nor
multi-wavelength images including ultra deep HST/WFC3 and Spitzer data. The SLE
has a significant line at 249.9 GHz with a signal-to-noise ratio of 7.1. If the
SLE is not a spurious source made by unknown systematic noise of ALMA, the
strong upper limits of our multi-wavelength data suggest that the SLE would be
a faint galaxy at z >~ 6.
