The ALMA REBELS Survey: The Cosmic HI Gas Mass Density in Galaxies at $z\approx 7$

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 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 $M_{\mathrm{HI}}/M_{\star }\approx 10$, corresponding to HI gas mass fractions of $f_{\mathrm{HI}}=M_{\mathrm{HI}}/(M_{\star }+M_{\mathrm{HI}})=90\%$, 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 (${\rho }_{\mathrm{HI}}$) at this redshift, which we measure to be ${\rho }_{\mathrm{HI}}=7.1_{-3.0}^{+6.4}\times 10^6{M}_{\odot }\mathrm{Mp}{\mathrm{c}}^{-3}$. This estimate is substantially lower by a factor of $\approx 10$ than that inferred from an extrapolation of damped Lyman-$\alpha$ absorber (DLA) measurements, and largely depend on the exact [CII] LF adopted. However, we find this decrease in ${\rho }_{\mathrm{HI}}$ to be consistent with recent simulations and argue that this apparent discrepancy is likely 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.

The accepted version of the manuscript in press for ApJL

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