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 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.

Morpheus Reveals Distant Disk Galaxy Morphologies with JWST: The First AI/ML Analysis of JWST Images

“…For instance, given the CANDELS stellar masses of our candidates are typically a billion solar masses, extrapolations of the the observed relationships between mass, rotational support, and redshift suggest that v/σ ∼ 4 might be expected (Kassin et al 2012;Turner et al 2017;Wisnioski et al 2019). However, less is known about the high-redshift end of our sample and recent Atacama Large Millimeter/Submillimeter Array (ALMA) observations, when combined with stellar population modeling, suggest that early star-forming main-sequence galaxies are extremely gas-rich (∼90%; Heintz et al 2022). ALMA observations of [C II] also show evidence of rotating disk galaxies as early as z ∼ 5.5 (Herrera-Camus et al 2022) and rotation in quasars as early as z ∼ 6.5 (Yue et al 2021).…”

The Gas and Stellar Content of a Metal-poor Galaxy at z = 8.496 as Revealed by JWST and ALMA

“…The Atacama Large Millimeter/submillimeter Array (ALMA) has been paramount in providing a clear census of the rest-frame far-infrared (FIR) emission of galaxies at z  6. The fine-structure 2 P 3/2 − 2 P 1/2 [C II] 158 μm line transition in particular, which is one of the major ISM cooling lines (Hollenbach & Tielens 1999;Wolfire et al 2003;Carilli & Walter 2013;Lagache et al 2018), has been used to infer the ISM dynamics (Capak et al 2015;Smit et al 2018;Matthee et al 2019) and overall gas content (Knudsen et al 2016;Dessauges-Zavadsky et al 2020;Heintz et al 2021Heintz et al , 2022aM. Aravena et al 2023, in preparation) of individual galaxies at z  6.…”

Investigating the [C ii]-to-H i Conversion Factor and the H i Gas Budget of Galaxies at z ≈ 6 with Hydrodynamic Simulations

“…C II is readily available in the high-redshift universe for large surveys (e.g., the Large ALMA-ALPINE survey at z ∼ 4-6; Béthermin et al 2020;Dessauges-Zavadsky et al 2020;Faisst et al 2020;Le Fèvre et al 2020, etc. ;and REBELS at z ∼ 6-8;e.g., Bouwens et al 2021;Heintz et al 2022). Establishing a proxy for H I is particularly important as even the next-generation radio facilities such as the Square Kilometre Array will only be able to detect and measure H I out to z ≈ 2 and will not be able to probe the evolution of H I in the universe beyond cosmic noon.…”