A high-redshift calibration of the [O I]-to-H I conversion factor in star-forming galaxies

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 HI 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 mu m and [O I]-145 mu m emission line luminosities and MHI in star-forming galaxies at z approximate to 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 MHI and [O I]-63 mu m and [O i]-145 mu m line luminosities in addition to the Sigame hydrodynamical simulation framework at similar epochs (z approximate to 0). We find that the [O I] 63 mu m-to-H i and [O I] 145 mu m-to-H I conversion factors, here denoted fi [OI] 63 mu m and fi [OI] 145 mu m, respectively, universally appear to be anti-correlated with the gas-phase metallicity. The GRB measurements further predict a mean ratio of L[OI]-63 mu m/L[OI]-145 mu m = 1:55 +/- 0:12 and reveal generally less excited [C II] over [OI] compared to the local galaxy sample. The z approximate to 0 galaxy sample also shows systematically higher beta([OI]-63 mu m) and beta([OI]-145) mu m conversion factors than the GRB sample, indicating either suppressed [OI] emission in local galaxies likely due to their lower hydrogen densities or more extended, diffuse HI gas reservoirs traced by the HI 21-cm. Finally, we apply these empirical calibrations to the few detections of [O i]-63 mu m and [O i]-145 mu m line transitions at z approximate to 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.