Atomic and molecular gas as traced by [C II] emission

The latest ALMA and JWST observations provide new information on the birth and evolution of galaxies in the early Universe at the epoch of reionization. Measurements at redshift z > 5 of their cold-gas budget are particularly important because this budget is known to be the main fuel for star formation. A powerful tool for probing the physics characterising galaxies at high redshift is the [C II ] 158 μm emission line. Due to its low excitation potential, [C II ] emission can be produced in photodissociation regions, neutral atomic gas, and molecular clouds. To properly capture the cold-gas processes taking place in these environments (molecule formation, self-shielding, dust grain catalysis, and photoelectric and cosmic-ray heating), we made use of a new set of dedicated hydrodynamic simulations (C OLD SIM) including time-dependent non-equilibrium chemistry, star formation, stellar evolution, metal spreading, and feedback mechanisms. We were able to accurately track the evolution of H I , H II and H 2 in a cosmological context and predict the contribution of each gas phase to [C II ] luminosity. We provide formulas that can be used to estimate the mass of molecular and atomic gas from [C II ] detections. Furthermore, we analysed the evolution of conversion factors with galactic properties, such as stellar metallicity, star formation rate, and stellar mass. We demonstrate that [C II ] emission is dominated by H I gas and that most of the [C II ] luminosity is generated in warm, dense, star-forming regions. We conclude that although [C II ] predominantly traces atomic rather than molecular gas, the [C II ] luminosity remains a robust indicator of the H 2 mass.