COLDSIM predictions of [C II] emission in primordial galaxies

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85748 Garching bei 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 bei München, Germany
⁵ Aix Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille (LAM) UMR 7326, 13388 Marseille, France

Received: 11 April 2024
Accepted: 1 June 2024

Abstract

Context. A powerful tool with which to probe the gas content at high redshift is the [C II] 158 μm submillimetre emission line, which, due to its low excitation potential and luminous emission, is considered a possible direct tracer of star forming gas.

Aims. In this work, we investigate the origin, evolution, and environmental dependencies of the [C II] 158 μm emission line, as well as its expected correlation with the stellar mass and star formation activity of the high-redshift galaxies observed by JWST.

Methods. We use a set of state-of-the-art cold-gas hydrodynamic simulations (COLDSIM) with fully coupled time-dependent atomic and molecular non-equilibrium chemistry and self-consistent [C II] emission from metal-enriched gas. We accurately track the evolution of H I, H II, and H₂ in a cosmological context and predict both global and galaxy-based [C II] properties.

Results. For the first time, we predict the cosmic mass density evolution of [C II] and find that it is in good agreement with new measurements at redshift z = 6 from high-resolution optical quasar spectroscopy. We find a correlation between [C II] luminosity, L_[C II], and stellar mass, which is consistent with results from ALMA high-redshift large programs. We predict a redshift evolution in the relation between L_[C II] and the star formation rate (SFR), and provide a fit to relate L_[C II] to SFR, which can be adopted as a more accurate alternative to the currently used linear relation.

Conclusions. Our findings provide physical grounds on which to interpret high-redshift detections in contemporary and future observations, such as the ones performed by ALMA and JWST, and to advance our knowledge of structure formation at early times.