SGAME Simulations of the [CII], [OI], and [OIII] Line Emission from Star-forming Galaxies at z≲ 6

Of the almost 40 star-forming galaxies at z ≥ 5 (not counting quasi-stellar objects) observed in [C II ] to date, nearly half are either very faint in [C II ] or not detected at all, and fall well below expectations based on locally derived relations between star formation rate and [C II ] luminosity. This has raised questions as to how reliable [C II ] is as a tracer of star formation activity at these epochs and how factors such as metallicity might affect the [C II ] emission. Combining cosmological zoom simulations of galaxies with SGAME (SImulator of GAlaxy Millimeter/ submillimeter Emission), we modeled the multiphased interstellar medium (ISM) and its emission in [C II ] , as well as in [O I ] and [O III ], from 30 main-sequence galaxies at z ≲6 with star formation rates ∼323 M· yr -1 , stellar masses ∼ 0.7 8 109 M·, and metallicities ∼ 0.1 0.4 Z?. The simulations are able to reproduce the aforementioned [C II ] faintness of some normal star-forming galaxy sources at z 5. In terms of [O I ] and [O III ], very few observations are available at z 5, but our simulations match two of the three existing z 5 detections of [O III ] and are furthermore roughly consistent with the [O I ] and [O III ] luminosity relations with star formation rate observed for local starburst galaxies. We find that the [C II ] emission is dominated by the diffuse ionized gas phase and molecular clouds, which on average contribute ∼66% and ∼27%, respectively. The molecular gas, which constitutes only ∼10% of the total gas mass, is thus a more efficient emitter of [C II ] than the ionized gas, which makes up ∼85% of the total gas mass. A principal component analysis shows that the [C II ] luminosity correlates with the star formation activity of a galaxy as well as its average metallicity. The low metallicities of our simulations together with their low molecular gas mass fractions can account for their [C II ] faintness, and we suggest that these factors may also be responsible for the [C II ] -faint normal galaxies observed at these early epochs.