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

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 ~ 3.5σ), 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 Mₘₒₗ = (1.0±0.4)×10¹¹\,(αCO/(3\, M₍K\,km\,s⁻¹\,pc⁻²)⁻¹))\,M_, directly confirming the presence of a very massive gas reservoir only 700\,Myr after the big bang. This implies an extreme gas fraction of fgas 0.95, a gas-to-dust ratio of δ GDR 6× 10², and a depletion time-scale of τ dep 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 Mₘₒₗ= (1.8+1.0-0.9)× 10¹¹\, M, independent of assumptions about r₃₁ and α 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-H₂ conversion factor of α _ [C\, II]=(60± 25)\,M$/$L$, 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 $z> 7$, 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.