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

Abstract

We present AC-2168, an almost NIRCam-dark, millimetre-bright galaxy in the COSMOS field. The source was identified blindly in ALMA Band-4 continuum data and remains undetected in the COSMOS-Web DR1 NIRCam catalogue. We spectroscopically confirm a redshift of $z_{\mathrm{spec}}=6.631$ from [CII] 158 $\hat{I}¼$m and four tentatively detected CO lines in NOEMA and ALMA data. SED fitting to near-IR to millimetre photometry yields ${\mathrm{L}}_{\mathrm{IR}}=1.6\times 10^{12}{\mathrm{L}}_{\odot }$, an SFR of $244{\mathrm{M}}_{\odot }/yr$, heavy dust attenuation ${\mathrm{A}}_{\mathrm{V}}=5.4$ mag, and a stellar mass ${\mathrm{M}}_{\star }=3.7\times 10^{10}{\mathrm{M}}_{\odot }$. From the millimetre continuum and [CII] emission, we infer a warm ISM with ${\mathrm{T}}_{\mathrm{dust}}=60K$, ${\mathrm{M}}_{\mathrm{dust}}=3.0\times 10^8{\mathrm{M}}_{\odot }$ and ${\mathrm{M}}_{\mathrm{gas}}=4.1\times 10^{10}{\mathrm{M}}_{\odot }$. AC-2168 has a gas fraction ($f_{\mathrm{gas}}=M_{\mathrm{gas}}/(M_{\star }+M_{\mathrm{gas}})$) of $\sim 52\%$, a short depletion time of $\sim 170Myr$, a compact ($\sim 1kpc$) dust-continuum size, and an SFR consistent with the star-forming main sequence at its mass. These properties match expectations for progenitors of massive quiescent galaxies at the peak of their assembly, as implied by NIRSpec-based SFHs of $z\sim 4-5$ systems. Using the blind detection, we estimate a space density of $7.8_{-6.5}^{+18.0}\times 10^{-6}\mathrm{cMp}{\mathrm{c}}^{-3}$ for AC-2168-like NIRCam-dark galaxies at $z\sim 6-7$, $\sim 42\%$ of the abundance of massive quiescent galaxies at $z\sim 4-5$. No overdensity of Ly$\hat{I}\pm$ emitters or Lyman-break galaxies is found nearby, suggesting AC-2168 does not lie in a prominent protocluster and highlighting the importance of unbiased blind surveys for this population.

12 pages, 6 figures, submitted