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 3. 5), 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 ₌₎₋ = (1. 00. 4) 10^11\, (₂₎/ (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 f ₆₀ₒ 0. 95, a gas-to-dust ratio of ₆₃ₑ 6 10², and a depletion time-scale of ₃₄ 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 ₌₎₋= (1. 8^+1. 0-₀. ₉) 10^11\, M, independent of assumptions about r₃₁ and ₂₎. 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 ₂\, ₈₈= (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.
Cescon et al. (Thu,) studied this question.