An almost NIRCam-dark dusty star-forming galaxy at z = 6.63

Abstract

We present AC-2168, an almost NIRCam-dark, millimetre-bright galaxy. The source was identified blindly in Atacama Large Millimeter/submillimeter Array (ALMA) Band-4 continuum data and remains undetected in the COSMOS-Web DR1 NIRCam catalogue. We spectroscopically confirm a redshift of (Formula presented) from [(Formula presented)] 158 (Formula presented) m and four tentatively detected CO lines in the data from the NOrthern Extended Millimeter Array (NOEMA) and ALMA. Modelling to near-IR to millimetre photometry yields (Formula presented), (Formula presented), (Formula presented), and (Formula presented) mag. From the millimetre continuum and [(Formula presented)] emission, we infer a warm interstellar medium with (Formula presented), (Formula presented), and (Formula presented). AC-2168 differs from normal UV-selected galaxies in its strong dust obscuration, yet its moderate star formation rate (SFR) at high (Formula presented), consistent with the star-forming main sequence, distinguishes it from most blindly detected dusty star-forming galaxies dominated by intense starbursts. Its short depletion time of (Formula presented), compact ((Formula presented)) dust-continuum size, and moderate SFR suggest a massive system caught in a rapid transition near the end of its main assembly phase, consistent with NIRSpec-based star formation histories of massive quiescent galaxies at (Formula presented). We also estimate a space density of (Formula presented) for AC-2168-like NIRCam-dark galaxies at (Formula presented), which reaches (Formula presented) of the abundance of massive quiescent galaxies at (Formula presented). These results jointly suggest that NIRCam-dark galaxies are likely the direct progenitors of a substantial fraction of massive quiescent galaxies.