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 } \sim 30 - 300 M _ { \odot } yr ^ { -1 } ) 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 } \lesssim 30 M _ { \odot } yr ^ { -1 } ) .
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 \lesssim 7 \times 10 ^ { 12 } M _ { \odot } , 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 \gtrsim 6 .