ALMA CENSUS OF FAINT 1.2mm SOURCES DOWN TO ∼ 0.02 mJy: EXTRAGALACTIC BACKGROUND LIGHT AND DUST-POOR, HIGH-z GALAXIES

We present statistics of 133 faint 1.2 mm continuum sources detected in about 120 deep Atacama Large Millimeter/submillimeter Array (ALMA) pointing data that include all the archival deep data available by 2015 June. We derive number counts of 1.2 mm continuum sources down to 0.02 mJy partly with the assistance of gravitational lensing, and find that the total integrated 1.2 mm flux of the securely identified sources is 22.9(-5.6)(+6.7) Jy deg(-2). which. corresponds to 104(-25)(+31)% of the extragalactic background light (EBL) measured by Cosmic Background Explorer observations. These results suggest that the major 1.2 mm EBL contributors are sources with less than or similar to 0.02 mJy, and that very faint 1.2 mm sources with. 0.02 mJy contribute negligibly to the EBL with the possible flattening and/or truncation of number counts in this very faint flux regime. To understand the physical origin of our faint ALMA sources, we measure the galaxy bias b(g) by the counts-in-cells technique, and place a stringent upper limit of b(g) < 3.5 that is not similar to b(g) values of massive distant red galaxies and submillimeter galaxies but comparable to those of UV-bright, star-forming BzK galaxies (sBzKs) and Lyman. break galaxies (LBGs). Moreover, in the optical and near-infrared (NIR) deep fields, we identify optical-NIR counterparts for 59% of our faint ALMA sources, the majority of which have luminosities, colors, and the IRX-beta relation the same as sBzKs and LBGs. We thus conclude that about a half of our faint ALMA sources are dust-poor, high-z galaxies as known as sBzKs and LBGs in optical studies, and that these faint ALMA sources are not miniature (U)LIRGs simply scaled down with the infrared brightness.