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Abstract

The [CII] 157.74 mu m fine structure transition is one of the brightest and most well-studied emission lines in the far-infrared (FIR), produced in the interstellar medium (ISM) of galaxies. We study its properties in sub-pc resolution hydrodynamical simulations for an ISM patch with gas surface density of Sigma(g)=10M(circle dot)pc(-2), coupled with time-dependent chemistry, far-ultraviolet (FUV) dust and gas shielding, star formation, photoionization and supernova (SN) feedback, and full line-radiative transfer. We find a [CII]-to-H-2 conversion factor that scales weakly with metallicity X-[CII]=6.31x10(19)Z '(0.17)cm(-2)(Kkms(-1))(-1), where Z ' is the normalized metallicity relative to solar. {The majority of [CII] originates from atomic gas with hydrogen number density n similar to 10 cm(-3).} The [CII] 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 similar to 0 observations. As such, [CII] 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)) interstellar medium (ISM) is important as it dominates [CII] 157.74 mu m emission. We compare our full radiative transfer computation with the optically-thin limit and find that the [CII] line becomes marginally optically thick only at super-solar metallicity for our assumed gas surface density.