[C II] Emission in a Self-regulated Interstellar Medium

The [C II] 157.74 μm fine-structure transition is one of the brightest and most well-studied emission lines in the far-infrared, produced in the interstellar medium (ISM) of galaxies. We study its properties in subparsec-resolution hydrodynamical simulations for an ISM patch with gas surface density of Σ _g = 10 M _⊙ pc^‑2, coupled with time-dependent chemistry, far-ultraviolet dust and gas shielding, star formation, photoionization and supernova feedback, and full line radiative transfer. We find a [C II]-to-H₂ conversion factor that scales weakly with metallicity ${X}_{[{\rm{C}}\,{\rm\small{II}}]}=6.31\times {10}^{19}\ {Z}^{{\prime} \ 0.17}\ {\mathrm{cm}}^{-2}\ {({\rm{K}}\ \mathrm{km}\ {{\rm{s}}}^{-1})}^{-1}$ , where ${Z}^{{\prime} }$ is the normalized metallicity relative to solar. The majority of [C II] originates from atomic gas with hydrogen number density n ∼ 10 cm^‑3. The [C II] line intensity positively correlates with the star formation rate (SFR), with a normalization factor that scales linearly with metallicity. We find that this is broadly consistent with z ∼ 0 observations. As such, [C II] is a good SFR tracer even in metal-poor environments where molecular lines might be undetectable. Resolving the clumpy structure of the dense (n = 10‑10³ cm^‑3) ISM is important, as it dominates [C II] 157.74 μm emission. We compare our full radiative transfer computation with the optically thin limit and find that the [C II] line becomes marginally optically thick only at supersolar metallicity for our assumed gas surface density.