Optical-Infrared Properties of Faint 1.3 mm Sources Detected with ALMA
  We report optical-infrared (IR) properties of faint 1.3 mm sources (S_1.3mm =
0.2-1.0 mJy) detected with the Atacama Large Millimeter/submillimeter Array
(ALMA) in the Subaru/XMM-Newton Deep Survey (SXDS) field. We searched for
optical/IR counterparts of 8 ALMA-detected sources (>=4.0 sigma, the sum of the
probability of spurious source contamination is ~1) in a K-band source catalog.
Four ALMA sources have K-band counterpart candidates within a 0.4" radius.
Comparison between ALMA-detected and undetected K-band sources in the same
observing fields shows that ALMA-detected sources tend to be brighter, more
massive, and more actively forming stars. While many of the ALMA-identified
submillimeter-bright galaxies (SMGs) in previous studies lie above the sequence
of star-forming galaxies in stellar mass--star-formation rate plane, our ALMA
sources are located in the sequence, suggesting that the ALMA-detected faint
sources are more like `normal' star-forming galaxies rather than `classical'
SMGs. We found a region where multiple ALMA sources and K-band sources reside
in a narrow photometric redshift range (z ~ 1.3-1.6) within a radius of 5" (42
kpc if we assume z = 1.45). This is possibly a pre-merging system and we may be
witnessing the early phase of formation of a massive elliptical galaxy.
