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

• Published in: Astronomy and astrophysics (Berlin) Vol. 685; p. A30
• Main Authors: Wilson, Sophia N., Heintz, Kasper E., Jakobsson, Páll, Madden, Suzanne C., Watson, Darach, Magdis, Georgios, Valentino, Francesco, Greve, Thomas R., Vizgan, David
• Format: Journal Article
• Language: English
• Published: Heidelberg EDP Sciences 01.05.2024
• Subjects: Atomic oxygen; Calibration; Emission; Far infrared radiation; Galactic evolution; Galaxies; Gamma ray bursts; Hydrogen; Interstellar gas; Interstellar matter; Ionization; Metallicity; Red shift; Star & galaxy formation; Star formation; Stars & galaxies
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202346878

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 S IGAME 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.