2017ApJ...846..105O

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 SIGAME (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 ∼3-23 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.