The ALMA REBELS Survey: The Cosmic H I Gas Mass Density in Galaxies at z ≈ 7
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 ${\rho }_{{\rm{H}}{\rm\small{I}}}={7.1}_{-3.0}^{+6.4}\times {10}^{6}\,{M}_{\odot }\,{\mathrm{Mpc}}^{-3}$ . 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2022
- DOI:
- 10.3847/2041-8213/ac8057
- arXiv:
- arXiv:2206.07763
- Bibcode:
- 2022ApJ...934L..27H
- Keywords:
-
- High-redshift galaxies;
- Galaxy evolution;
- Interstellar medium;
- 734;
- 594;
- 847;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- The accepted version of the manuscript in press for ApJL