Extremely Faint Millimeter Sources Identified by Multi-Field Deep ALMA Survey and Subaru Follow-up Spectroscopy

Seiji Fujimoto

We present statistics of 133 faint 1.2-mm continuum sources detected in 67 ALMA Band 6/7 maps that include all the archival deep data available by 2015 June. We derive number counts at 1.2 mm down to the extremely faint flux limit of 0.02 mJy in conjunction with gravitational lensing magnifications, and almost fully resolve the cosmic infrared background light. To understand the physical origin of our faint ALMA sources, we measure the galaxy bias (bg), and place a stringent upper limit of bg < 3.5 that is comparable to those of sBzKs and LBGs. We identify optical-NIR counterparts for 59% of our faint ALMA sources. The majority of the optical-NIR counterparts have luminosities, colors, and the IRX-beta relation same as sBzKs and LBGs, suggesting that about a half of our faint ALMA sources are typical high-z galaxies found in optical studies. Our recent Subaru/MOIRCS follow-up spectroscopy successfullydetect Ha and/or [OIII]5007 emission of our two strongly-lensed ALMA sourcesat z = 2.63 and 2.50 with intrinsic 1mm fluxes of 002-0.03 mJy. We compare the ISM propertiesof these galaxies, gas-phase metallicity and dust mass, with those of bright (>~1 mJy)millimeter galaxies, and discuss the physical origin of the faint millimeter emissionof high-z galaxies.