{
  "id": "2206.07763",
  "version": "v1",
  "published": "2022-06-15T18:56:55.000Z",
  "updated": "2022-06-15T18:56:55.000Z",
  "title": "The ALMA REBELS Survey: The Cosmic HI Gas Mass Density in Galaxies at $z\\approx 7$",
  "authors": [
    "K. E. Heintz",
    "P. A. Oesch",
    "M. Aravena",
    "R. J. Bouwens",
    "P. Dayal",
    "A. Ferrara",
    "Y. Fudamoto",
    "L. Graziani",
    "H. Inami",
    "L. Sommovigo",
    "R. Smit",
    "M. Stefanon",
    "M. Topping",
    "A. Pallottini",
    "P. van der Werf"
  ],
  "comment": "Resubmitted to ApJL after taking into account the comments from the referee",
  "categories": [
    "astro-ph.GA",
    "astro-ph.CO"
  ],
  "abstract": "The neutral atomic gas content of individual galaxies at large cosmological distances has until recently been difficult to measure due to the weakness of the hyperfine HI 21-cm transition. Here we estimate the HI gas mass of a sample of main-sequence star-forming galaxies at $z\\sim 6.5 - 7.8$ surveyed for [CII]$-158\\mu$m emission as part of the Reionization Era Bright Emission Line Survey (REBELS), using a recent calibration of the [CII]-to-HI conversion factor. We find that the HI gas mass excess in galaxies increases as a function of redshift, with an average of $M_{\\rm HI} / M_\\star \\approx 10$, corresponding to HI gas mass fractions of $f_{\\rm HI} = M_{\\rm HI} / (M_\\star + M_{\\rm HI}) = 90\\%$, at $z\\approx 7$. Based on the [CII]-$158\\mu$m luminosity function (LF) derived from the same sample of galaxies, we further place constraints on the cosmic HI gas mass density in galaxies ($\\rho_{\\rm HI}$) at this redshift, which we measure to be $\\rho_{\\rm HI} = 7.1^{+6.4}_{-3.0} \\times 10^{6}\\,M_{\\odot}\\,{\\rm Mpc^{-3}}$. This estimate is substantially lower by a factor of $\\approx 10$ than that inferred from an extrapolation of damped Lyman-$\\alpha$ absorber (DLA) measurements. We argue that this apparent discrepancy is a consequence of the DLA sightlines predominantly probing the substantial fraction of HI gas in high-$z$ galactic halos, whereas [CII] traces the HI in the ISM associated with star formation. We make predictions for this build-up of neutral gas in galaxies as a function of redshift, showing that at $z\\gtrsim 5$ only $\\approx 10\\%$ of the cosmic HI gas content is confined in galaxies and associated with the star-forming ISM.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2022-06-15T18:56:55.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "cosmic hi gas mass density",
      "alma rebels survey",
      "bright emission line survey",
      "era bright emission line"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}