C ii Emission in a Self-regulated Interstellar Medium

Abstract The C ii 157.74 μ m 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-H 2 conversion factor that scales weakly with metallicity X C II = 6.31 × 10 19 Z ′ 0.17 cm − 2 ( K km s − 1 ) − 1 , where Z ′ 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 = 10−10 3 cm −3 ) ISM is important, as it dominates C ii 157.74 μ m 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.