Synthetic [C ii] emission maps of a simulated molecular cloud in formation

The C+ ion is an important coolant of interstellar gas, and so the [C ii] fine structure line is frequently observed in the interstellar medium. However, the physical and chemical properties of the [C ii]-emitting gas are still unclear. We carry out non-LTE (local thermal equilibrium) radiative transfer simulations with radmc-3d to study the [C ii] line emission from a young, turbulent molecular cloud before the onset of star formation, using data from the SILCC-Zoom project. The [C ii] emission is optically thick over 40 per cent of the observable area with I[C ii] > 0.5 K km s−1. To determine the physical properties of the [C ii] emitting gas, we treat the [C ii] emission as optically thin. We find that the [C ii] emission originates primarily from cold, moderate density gas (40 ≲ T ≲ 65 K and 50 ≲ n ≲ 440 cm−3), composed mainly of atomic hydrogen and with an effective visual extinction between ∼0.50 and ∼0.91. Gas dominated by molecular hydrogen contributes only ≲20 per cent of the total [C ii] line emission. Thus, [C ii] is not a good tracer for CO–dark H2 at this early phase in the cloud’s lifetime. We also find that the total gas, H and C+ column densities are all correlated with the integrated [C ii] line emission, with power law slopes ranging from 0.5 to 0.7. Further, the median ratio between the total column density and the [C ii] line emission is YC ii ≈ 1.1 × 1021 cm−2 (K km s−1)−1, and |Y_ C\, II| scales with |I_[C\, II]-0.3|⁠. We expect |Y_ C\, II| to change in environments with a lower or higher radiation field than simulated here.