2025ApJ...987L...2H

At cosmic dawn, the first stars and galaxies are believed to form from and be deeply embedded in clouds of dense, pristine gas. Here we present a study of the James Webb Space Telescope/NIRSpec data of the most distant, spectroscopically confirmed galaxy observed to date, JADES-GS-z14-0 (GS-z14 for short), at z = 14.179, combined with recently reported far-infrared measurements of the [O III]-88 μm and [C II]-158 μm line transitions and underlying dust-continuum emission. Based on the observed prominent damped Lyα (DLA) absorption profile, we determine a substantial neutral atomic hydrogen (H I) column density, $\mathrm{log}({N}_{{\rm{HI}}}/{{\rm{cm}}}^{-2})=22.2{7}_{-0.09}^{+0.08}$, consistent with previous estimates though seemingly at odds with the dynamical and gas mass of the galaxy. Using various independent but complementary approaches, considering the implied neutral gas mass from the DLA measurement, the star formation rate surface density, and the metal abundance, we demonstrate that the total gas mass of GS-z14 is of the order M_gas = 10^9.5-10^9.8M_⊙. This implies a substantial gas mass fraction, f_gas ≈ 0.7-0.9 and that the bulk of the interstellar medium (ISM) is in the form of H I, with mass ratios ${M}_{{\rm{HI}}}/{M}_{{{\rm{H}}}_{2}}\approx 3$. We show that the derived gas mass is fully consistent with the nondetection of [C II]-158 μm, assuming an appropriate scaling to the neutral gas. The low dust-to-gas ratio, A_V/N_HI = (1.3 ± 0.6) × 10^–23 mag cm², derived in the line of sight through the DLA further indicates that the absorbing gas is more pristine than the central, star-forming regions probed by the [O III]-88 μm emission. These results highlight the implications for far-infrared line-detection searchers attainable with the Atacama Large Millimeter/submillimeter Array and demonstrate that the bright, relatively massive galaxy GS-z14 at z = 14.179 is deeply embedded in a substantial, pristine H I gas reservoir dominating its baryonic matter content.