Investigating the [C$\,{\rm \scriptsize II}$]-to-H$\,{\rm \scriptsize I}$ conversion factor and the H$\,{\rm \scriptsize I}$ gas budget of galaxies at $z\approx 6$ with hydrodynamical simulations

TL;DR

This study uses hydrodynamical simulations to evaluate the [CII] 158μm line as a proxy for neutral hydrogen in high-redshift galaxies, revealing a strong correlation and insights into the baryonic composition at z≈6.

Contribution

It demonstrates the relationship between [CII] emission and H I gas in simulated galaxies at z≈6, providing a new method to estimate H I content in distant galaxies.

Findings

Strong correlation between [CII] and H I gas content.

Anti-correlation of [CII]-to-H I relation with metallicity.

H I is the dominant baryonic component in galaxies at z≈6.

Abstract

One of the most fundamental baryonic matter components of galaxies is the neutral atomic hydrogen (H$\,{\rm \scriptsize I}$). At low redshifts, this component can be traced directly through the 21-cm transition, but to infer H$\,{\rm \scriptsize I}$ gas content of the most distant galaxies, a viable tracer is needed. We here investigate the fidelity of the fine structure transition of the ($^2P_{3/2} - ^2P_{1/3}$) transition of singly-ionized carbon [C$\,{\rm \scriptsize II}$] at $158\,\mu$m as a proxy for H$\,{\rm \scriptsize I}$ in a set simulated galaxies at $z\approx 6$, following the work by Heintz et al. (2021). We select 11,125 star-forming galaxies from the SIMBA simulations, with far-infrared line emissions post-processed and modeled within the SIGAME framework. We find a strong connection between [C$\,{\rm \scriptsize II}$] and H$\,{\rm \scriptsize I}$, with the relation between this [C$\,{\rm \scriptsize II}$]-to-H$\,{\rm \scriptsize I}$ relation ($\beta_{\rm [C\,{\rm \scriptsize II}]}$) being anti-correlated with the gas-phase metallicity of the simulated galaxies. We further use these simulations to make predictions for the total baryonic matter content of galaxies at $z\approx 6$, and specifically the HI gas mass fraction. We find mean values of $M_{\rm HI}/M_\star = 1.4$, and $M_{\rm HI}/M_{\rm bar,tot} = 0.45$. These results provide strong evidence for H$\,{\rm \scriptsize I}$ being the dominant baryonic matter component by mass in galaxies at $z\approx 6$.