2023A&A...677A.172K

We present NOEMA CO (2-1) line and ALMA 870 µm continuum observations of a main-sequence galaxy at z = 1.38. The galaxy was initially deemed a “gas giant” based on the gas mass derived from sub-mm continuum (log(M_gas/M_⊙) = 11.20 ± 0.20), however, the gas mass derived from CO (2-1) luminosity brings the gas mass value down to a level that is consistent with typical values for star-forming galaxies at that redshift (log(M_gas/M_⊙) = 10.84 ± 0.03). Meanwhile, the dust-to-stellar mass ratio remains elevated above the scaling relations by a factor of 5. In this work, we explore the potential physical picture and consider an underestimated stellar mass and optically thick dust as possible explanations. Based on the updated gas-to-stellar mass ratio, we can rule out the former; while the latter may indeed contribute to the overestimation of the dust mass, it is not sufficient enough to explain the observed physical picture overall. Instead, other plausible explanations include enhanced HI reservoirs, an unusually high metallicity, or the presence of an optically dark, dusty contaminant. Using the ALMA data at 870 µm coupled with HST/ACS imaging, we find an extended morphology in dust continuum and clumpy star-formation in rest-frame UV in this galaxy. In addition, a tentative ∼10 kpc dusty arm is found to be bridging the galaxy center and a clump in F814W image. The galaxy shows levels of dust obscuration similar to the so-called HST-dark galaxies at higher redshifts, thus falling into the optically faint and dark JWST color-color selection at z > 2. It is therefore possible that our object may stand as a low-z analog of the HST-dark populations. This galaxy serves as a caveat to the gas masses based on the continuum alone, with a larger sample required to unveil the full picture.