{"about":{"site":"https://codewithpapers.app","non_affiliation":"Code with Papers and Syntology are not affiliated with, endorsed by, or sponsored by Papers with Code, Meta, or the pwc-archive mirror.","licence":"CC BY-SA 4.0","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","attribution":"https://codewithpapers.app/attribution","modified":"archive material modified by Syntology; see the attribution page"},"url":"/paper/measuring-the-hi-content-of-individual","title":"Measuring the HI content of individual galaxies out to the epoch of reionization with [CII]","arxiv_id":"2108.13442","date":"2021-08-30","proceeding":null,"authors":["Kasper E. Heintz","Darach Watson","Pascal Oesch","Desika Narayanan","Suzanne C. Madden"],"abstract":"The HI gas content is a key ingredient in galaxy evolution, the study of which has been limited to moderate cosmological distances for individual galaxies due to the weakness of the hyperfine HI 21-cm transition. Here we present a new approach that allows us to infer the HI gas mass $M_{\\rm HI}$ of individual galaxies up to $z\\approx 6$, based on a direct measurement of the [CII]-to-HI conversion factor in star-forming galaxies at $z\\gtrsim 2$ using $\\gamma$-ray burst afterglows. By compiling recent [CII]-158 $\\mu$m emission line measurements we quantify the evolution of the HI content in galaxies through cosmic time. We find that the HI mass starts to exceed the stellar mass $M_\\star$ at $z\\gtrsim 1$, and increases as a function of redshift. The HI fraction of the total baryonic mass increases from around $20\\%$ at $z = 0$ to about $60\\%$ at $z\\sim 6$. We further uncover a universal relation between the HI gas fraction $M_{\\rm HI}/M_\\star$ and the gas-phase metallicity, which seems to hold from $z\\approx 6$ to $z=0$. The majority of galaxies at $z>2$ are observed to have HI depletion times, $t_{\\rm dep,HI} = M_{\\rm HI}/{\\rm SFR}$, less than $\\approx 2$ Gyr, substantially shorter than for $z\\sim 0$ galaxies. Finally, we use the [CII]-to-HI conversion factor to determine the cosmic mass density of HI in galaxies, $\\rho_{\\rm HI}$, at three distinct epochs: $z\\approx 0$, $z\\approx 2$, and $z\\sim 4-6$. These measurements are consistent with previous estimates based on 21-cm HI observations in the local Universe and with damped Lyman-$\\alpha$ absorbers (DLAs) at $z\\gtrsim 2$, suggesting an overall decrease by a factor of $\\approx 5$ in $\\rho_{\\rm HI}(z)$ from the end of the reionization epoch to the present.","url_abs":"https://arxiv.org/abs/2108.13442v1","url_pdf":"https://arxiv.org/pdf/2108.13442v1.pdf","source":{"archive":"pwc-archive (Hugging Face), CC BY-SA 4.0","snapshot":"2025-07-28","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","row_kind":"links_only","authors_date_abstract":"arXiv metadata, CC0 1.0 (https://info.arxiv.org/help/license), from the Kaggle arXiv metadata snapshot of 2026-09-12"},"code_links":[{"paper_slug":"measuring-the-hi-content-of-individual","repo_url":"https://github.com/jkrogager/VoigtFit","is_official":1,"mentioned_in_paper":1,"mentioned_in_github":0,"framework":"none","reach":null}],"tasks":[],"methods":[],"datasets_introduced":[],"methods_introduced":[],"results":[],"syntology":{"syntology_url":null,"atlas_url":null,"mcp":null,"developers":"https://syntology.ai/developers"},"arxiv_metadata":null,"syntology_extracted_results":null}