Faint End of 1.3 mm Number Counts Revealed by ALMA

We present the faint end of number counts at 1.3 mm (238 GHz) obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Band 6 observations were carried out targeting 20 star-forming galaxies at z ~ 1.4 in the Subaru/XMM-Newton Deep Survey field. In the observations, we serendipitously detect 15 sources (>=3.8σ, S _{1.3 mm} = 0.15-0.61 mJy) other than the targeted sources. We create number counts by using these "sub-mJy sources," which probe the faintest flux range among surveys at millimeter wavelengths. The number counts are consistent with (flux-scaled) number counts at 850 μm and 870 μm obtained with gravitational lensing clusters. The ALMA number counts agree well with model predictions, which suggest that these sub-mJy populations are more like "normal" star-forming galaxies than "classical" submillimeter galaxies with intense star-forming activity. In this flux range, ~80% of the extragalactic background light at 1.3 mm is resolved into individual sources.