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 $z_{\rm spec}=6.631$ from [${\rm C\, {\small II}}$] 158 $\mu$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 $L_{\rm IR}=1.6^{+0.6}_{-0.4}\times 10^{12}\, \mathrm{L}_\odot$, ${\rm SFR}=244^{+82}_{-55}\, \mathrm{M}_\odot \, \rm yr^{-1}$, $M_\star =3.7^{+12.2}_{-2.6}\times 10^{10}\, \mathrm{M}_\odot$, and $A_{\rm V}=5.4\pm 1.6$ mag. From the millimetre continuum and [${\rm C\, {\small II}}$] emission, we infer a warm interstellar medium with $T_{\rm dust}=60\pm 11K$, $M_{\rm dust}=3.0^{+0.8}_{-0.5}\times 10^{8}\, \mathrm{M}_\odot$, and $M_{\rm gas}=4.1\pm 0.8\times 10^{10}\, \mathrm{M}_\odot$. AC-2168 differs from normal UV-selected galaxies in its strong dust obscuration, yet its moderate star formation rate (SFR) at high $M_\star$, 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 $\sim 170\, \rm Myr$, compact ($\sim 1\, \rm kpc$) 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 $z\sim 4-5$. We also estimate a space density of $\rm 7.8^{+18.0}_{-6.5}\times 10^{-6}\, cMpc^{-3}$ for AC-2168-like NIRCam-dark galaxies at $z\sim 6-7$, which reaches $\sim 42\hbox{ per cent}$ of the abundance of massive quiescent galaxies at $z\sim 4-5$. These results jointly suggest that NIRCam-dark galaxies are likely the direct progenitors of a substantial fraction of massive quiescent galaxies.