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

Heintz, KE, Oesch, PA, Aravena, M, Bouwens, RJ, Dayal, P, Ferrara, A, Fudamoto, Y, Graziani, L, Inami, H, Sommovigo, L, Smit, R ORCID: 0000-0001-8034-7802, Stefanon, M, Topping, M, Pallottini, A and van der Werf, P (2022) The ALMA REBELS Survey: The Cosmic H i Gas Mass Density in Galaxies at z ≈ 7. Astrophysical Journal Letters, 934 (2). ISSN 2041-8205

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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 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.

Item Type:	Article
Uncontrolled Keywords:	0201 Astronomical and Space Sciences; Astronomy & Astrophysics
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	American Astronomical Society; IOP Publishing
Date of acceptance:	12 July 2022
Date of first compliant Open Access:	8 November 2022
Date Deposited:	08 Nov 2022 14:13
Last Modified:	05 Jul 2025 12:45
DOI or ID number:	10.3847/2041-8213/ac8057
URI:	https://researchonline.ljmu.ac.uk/id/eprint/18072

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