The ALMA REBELS Survey: The Cosmic H i Gas Mass Density in Galaxies at z ≈ 7

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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)


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 H i 21 cm transition. Here we estimate the H i gas mass of a sample of main-sequence star-forming galaxies at z ∼ 6.5-7.8 surveyed for [C ii] 158 μm emission as part of the Reionization Era Bright Emission Line Survey (REBELS), using a recent calibration of the [C ii]-to-H i conversion factor. We find that the H i gas mass excess in galaxies increases as a function of redshift, with an average of M Hi /M ≈ 10, corresponding to H i gas mass fractions of f Hi = M Hi /(M + M Hi ) = 90%, at z ≈ 7. Based on the [C ii] 158 μm luminosity function (LF) derived from the same sample of galaxies, we further place constraints on the cosmic H i gas mass density in galaxies (ρ Hi ) at this redshift, which we measure to be ρ H I = 7.1 − 3.0 + 6.4 × 10 6 M ⊙ Mpc − 3 . This estimate is substantially lower by a factor of ≈10 than that inferred from an extrapolation of damped Lyα absorber (DLA) measurements and largely depends on the exact [C ii] LF adopted. However, we find this decrease in ρ Hi to be consistent with recent simulations and argue that this apparent discrepancy is likely a consequence of the DLA sight lines predominantly probing the substantial fraction of H i gas in high-z galactic halos, whereas [C ii] traces the H i in the ISM associated with star formation. We make predictions for this buildup of neutral gas in galaxies as a function of redshift, showing that at z ≳ 5, only ≈10% of the cosmic H i gas content is confined in galaxies and associated with the star-forming ISM.

Original languageEnglish
Article numberL27
JournalAstrophysical Journal Letters
Issue number2
Publication statusPublished - 2022 Aug 1

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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