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The neutral atomic gas content of individual galaxies at large cosmologicaldistances has until recently been difficult to measure due to the weakness ofthe hyperfine HI 21-cm transition. Here we estimate the HI gas mass of a sampleof 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 LineSurvey (REBELS), using a recent calibration of the [CII]-to-HI conversionfactor. We find that the HI gas mass excess in galaxies increases as a functionof redshift, with an average of $M_{\rm HI} / M_\star \approx 10$,corresponding to HI gas mass fractions of $f_{\rm HI} = M_{\rm HI} / (M_\star +M_{\rm HI}) = 90\%$, at $z\approx 7$. Based on the [CII]-$158\mu$m luminosityfunction (LF) derived from the same sample of galaxies, we further placeconstraints on the cosmic HI gas mass density in galaxies ($\rho_{\rm HI}$) atthis redshift, which we measure to be $\rho_{\rm HI} = 7.1^{+6.4}_{-3.0} \times10^{6}\,M_{\odot}\,{\rm Mpc^{-3}}$. This estimate is substantially lower by afactor of $\approx 10$ than that inferred from an extrapolation of dampedLyman-$\alpha$ absorber (DLA) measurements, and largely depend on the exact[CII] LF adopted. However, we find this decrease in $\rho_{\rm HI}$ to beconsistent with recent simulations and argue that this apparent discrepancy islikely a consequence of the DLA sightlines predominantly probing thesubstantial fraction of HI gas in high-$z$ galactic halos, whereas [CII] tracesthe HI in the ISM associated with star formation. We make predictions for thisbuild-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 ingalaxies and associated with the star-forming ISM.

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