Croxall, K VSmith, J. D.Pellegrini, E.Groves, BrentBolatto, AlbertoHerrera-Camus, RodrigoSandstrom, Karin MDraine, Bruce TWolfire, M.Armus, LeeBoquien, MédéricBrandl, BDale, Daniel AGalametz, MaudHunt, LeslieKennicutt, Robert CKreckel, K.Rigopoulou, D.Van der Werf, P.Wilson, Christine D.2021-09-300004-637Xhttp://hdl.handle.net/1885/249095The [C ii] 158 μm fine-structure line is the brightest emission line observed in local star-forming galaxies. As a major coolant of the gas-phase interstellar medium, [C ii] balances the heating, including that due to far-ultraviolet photons, which heat the gas via the photoelectric effect. However, the origin of [C ii] emission remains unclear because C+ can be found in multiple phases of the interstellar medium. Here we measure the fractions of [C ii] emission originating in the ionized and neutral gas phases of a sample of nearby galaxies. We use the [N ii] 205 μm fine-structure line to trace the ionized medium, thereby eliminating the strong density dependence that exists in the ratio of [C ii]/[N ii] 122 μm. Using the FIR [C ii] and [N ii] emission detected by the KINGFISH (Key Insights on Nearby Galaxies: a Far- Infrared Survey with Herschel) and Beyond the Peak Herschel programs, we show that 60%–80% of [C ii] emission originates from neutral gas. We find that the fraction of [C ii] originating in the neutral medium has a weak dependence on dust temperature and the surface density of star formation, and has a stronger dependence on the gas-phase metallicity. In metal-rich environments, the relatively cooler ionized gas makes substantially larger contributions to total [C ii] emission than at low abundance, contrary to prior expectations. Approximate calibrations of this metallicity trend are providedK.V.C. acknowledges support from the Deutsche Forschungsgemeinschaft Priority Program ISM-SPP 1573 and the MPIA, and thanks the KINGFISH & BTP teams for their support. J.D.S. acknowledges visiting support from the Alexander von Humboldt Foundation and the MPIA. B.G. acknowledges the support of the Australian Research Council via a Future Fellowship (FT140101202). A.D.B. acknowledges partial support from NSFAST0955836 and 1412419. B.T.D. acknowledges partial support from NSF-AST1408723. M.B. was supported by MINEDUC-UA project, code ANT 1655. Beyond the Peak research has been supported by a NASA/JPL grant (RSA 1427378).application/pdfen-AU© 2017. The American Astronomical Society.galaxies: ISMISM: lines and bands201710.3847/1538-4357/aa80352020-11-23