2024A&A...685A..30W

The assembly and build-up of neutral atomic hydrogen (H I) in galaxies is one of the most fundamental processes in galaxy formation and evolution. Studying this process directly in the early universe is hindered by the weakness of the hyperfine 21-cm H I line transition, impeding direct detections and measurements of the H I gas masses (M_HI). Here we present a new method to infer M_HI of high-redshift galaxies using neutral, atomic oxygen as a proxy. Specifically, we derive metallicity-dependent conversion factors relating the far-infrared [O I]-63 μm and [O I]-145 μm emission line luminosities and M_HI in star-forming galaxies at z ≈ 2 - 6 using gamma-ray bursts (GRBs) as probes. We calibrate the [O I]-to-H I conversion factor relying on a sample of local galaxies with direct measurements of M_HI and [O I]-63 μm and [O I]-145 μm line luminosities in addition to the SIGAME hydrodynamical simulation framework at similar epochs (z ≈ 0). We find that the [O I]_63 μm_-to-H I and [O I]_145 μm_-to-H I conversion factors, here denoted β_[OI]-63 μm_ and β_[OI]-145 μm_, respectively, universally appear to be anti-correlated with the gas-phase metallicity. The GRB measurements further predict a mean ratio of L_[OI]-63 μm_/L_[OI]-145 μm_ = 1.55 ± 0.12 and reveal generally less excited [C II] over [O I] compared to the local galaxy sample. The z ≈ 0 galaxy sample also shows systematically higher β_[OI]-63 μm_ and β_[OI]-145 μm_ conversion factors than the GRB sample, indicating either suppressed [O I] emission in local galaxies likely due to their lower hydrogen densities or more extended, diffuse H I gas reservoirs traced by the H I 21-cm. Finally, we apply these empirical calibrations to the few detections of [O I]-63 μm and [O I]-145 μm line transitions at z ≈ 2 from the literature and further discuss the applicability of these conversion factors to probe the H I gas content in the dense, star-forming interstellar medium (ISM) of galaxies well into the epoch of reionization.