Oral Presentation

ALMA Lensing Cluster Survey: Physical properties of intrinsically faint near-infrared dark galaxies at z~3-4

Author(s): Akiyoshi Tsujita (U. Tokyo), Kotaro Kohno (U. Tokyo), Bunyo Hatsukade (U. Tokyo), Fumi Egusa (U. Tokyo)

Presenter: Akiyoshi Tsujita (The University of Tokyo)

Since the advent of ALMA, not only the extreme star-forming galaxies (classical SMGs) but also fainter (S_1.2mm < 1 mJy), that is, more common ones have been investigated in the early Universe. Recently, it has been revealed that some of these fainter ALMA objects are undetectable in deep near-infrared imaging with HST due to strong dust attenuation, called NIR-dark sources, and have been missed in LBG selections. Although this population is thought to be important for understanding the formation of massive galaxies, there have been few NIR-dark sources with confirmed spectroscopic redshift due to the difficulty of observations.
We performed ALMA band 3/4 follow-up line scan observations toward two faint NIR-dark sources in two massive clusters, MACS J0417.5-1154 and RXC J0032.1+1808, which are detected by ALMA Lensing Cluster Survey, a 1.2 mm survey in the 33-lensing cluster regions. We successfully detected CO emission lines and confirmed their spectroscopic redshifts to be z=3.652 and 2.985. We constrained their physical parameters such as stellar mass, star formation rate, gas depletion time, and size of dust-emitting region utilizing rich archival data from HST, Spitzer, and Herschel. One of the sources has a compact dust-emitting region (Re~0.8 kpc) and exhibits high star formation efficiency and far-infrared surface brightness, similar to classical SMGs and previously reported (mm-bright) NIR-dark sources with spectroscopic redshifts, which can be interpreted to be in a merger phase. In contrast, the other one has an extended dust emitting region (Re~2.0 kpc) and exhibits modest star formation efficiency and far-infrared surface brightness, as well as an elongated projected axis ratio. These results suggest that normal dusty star-forming galaxies with edge-on disks can be NIR-dark and may have been missed in previous observations. This is also consistent with recent findings from early JWST observations (Nelson et al. 2022).