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

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching b. München, Germany
² INAF-Italian National Institute of Astrophysics, Observatory of Trieste, via G. Tiepolo 11, 34143 Trieste, Italy
³ IFPU-Institute for Fundamental Physics of the Universe, via Beirut 2, 34014 Trieste, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
⁵ Aix Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille (LAM) UMR 7326, 13388 Marseille, France

^⋆ Corresponding author; benecasa@mpa-garching.mpg.de

Received: 17 September 2024
Accepted: 2 December 2024

Abstract

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 (COLDSIM) 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 H₂ 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 H₂ mass.