The Origins of [C II] Emission in Local Star-forming Galaxies

K. V. Croxall, J. D. Smith, E. Pellegrini, B. Groves, A. Bolatto, R. Herrera-Camus, K. M. Sandstrom, B. Draine, M. G. Wolfire, L. Armus, M. Boquien, B. Brandl, D. Dale, M. Galametz, L. Hunt, R. Kennicutt, K. Kreckel, D. Rigopoulou, P. Van Der Werf, C. Wilson

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The [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 provided.

Original languageEnglish (US)
Article number96
JournalAstrophysical Journal
Issue number2
StatePublished - Aug 20 2017

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • ISM: lines and bands
  • galaxies: ISM


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