COLDSIM predictions of C II emission in primordial galaxies

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 (C OLD SIM) 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 H 2 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.