Direct detection of cool molecular gas in a star-forming galaxy at z =7.31

ABSTRACT We investigate the molecular gas content and interstellar medium (ISM) conditions of REBELS-25, a massive, star-forming galaxy at $z=7.31$. Deep VLA (Very Large Array) Q-band and Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 observations reveal CO(3−2) and CO(7−6) emission (both at $\sim 3.5\sigma$), and provide an upper limit on [C i](2−1). From the CMB (cosmic microwave background)-corrected CO(3−2) flux – representing the highest redshift detection of a low-J CO transition to date – we derive a molecular gas mass of $M_{\rm mol} = (1.0\pm0.4)\times10^{11}\,(\alpha_{\rm CO}/(3\, \rm{M}_\odot(\rm{K\,km\,s^{-1}\,pc^{-2})^{-1}))\,M_\odot}$, directly confirming the presence of a very massive gas reservoir only $\simeq 700\,$Myr after the big bang. This implies an extreme gas fraction of $f_{\rm gas}\simeq 0.95$, a gas-to-dust ratio of $\delta _{\rm GDR}\simeq 6\times 10^2$, and a depletion time-scale of $\tau _{\rm dep}\simeq 1.2\,$ Gyr, broadly consistent with extrapolated scaling relations for main-sequence galaxies at lower redshift. Using the radiative transfer code tuner, we self-consistently model CO and dust continuum emission in the context of the significant CMB background, constraining ISM properties and recovering $M_{\rm mol}= (1.8^{+1.0}_{-0.9})\times 10^{11}\, \mathrm{M}_{\odot }$, independent of assumptions about $r_{31}$ and $\alpha _{\rm CO}$. We further discuss the use of alternative molecular gas tracers at early epochs. Combining CO and [C ii] measurements, we infer an empirical [C ii]-to-H$_2$ conversion factor of $\alpha _{\rm [C\, {\small II}]}=(60\pm 25)\,$$\mathrm{M}_{\odot }$/$\mathrm{L}_{\odot }$, suggesting [C ii] remains a viable molecular gas tracer in the epoch of reionization. These results demonstrate the detectability of low-J CO emission even at $z\gt 7$, paving the way for next-generation facilities, and provide critical insights into the rapid mass assembly of galaxies during the first billion years of cosmic history.