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 Q-band and 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-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 $M_{\mathrm{mol}}=(1.0\pm 0.4)\times 10^{11}({\alpha }_{\mathrm{CO}}/(3$M${}_{\odot }$(K\kmspc${}^2{)}^{-1}))$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 $f_{\mathrm{gas}}\simeq 0.95$, a gas-to-dust ratio of ${\delta }_{\mathrm{GDR}}\simeq 6\times 10^2$, and a depletion timescale of ${\tau }_{\mathrm{dep}}\simeq 1.2$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 $M_{\mathrm{mol}}=(1.8_{-0.9}^{+1.0})\times 10^{11}$M${}_{\odot }$, independent of assumptions about $r_{31}$ and ${\alpha }_{\mathrm{CO}}$. 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 ${\alpha }_{[\mathrm{CII}]}=(60\pm 25)$M${}_{\odot }$/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 $z>7$, 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.

Accepted for publication in MNRAS. Main text: 17 pages, 6 figures

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