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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.