The Origins of [C II] Emission in Local Star-forming Galaxies
K. V. Croxall (Illumination Works LLC, The Ohio State University, Max-Planck-Institut für Astronomie)
J. D T Smith (Max-Planck-Institut für Astronomie, University of Toledo)
E. Pellegrini (University of Toledo, University of Heidelberg)
Brent Groves (Australian National University)
Alberto Bolatto (University of Maryland)
Rodrigo Herrera-Camus (Max Planck Institute for Extraterrestrial Physics Garching)
K. M. Sandstrom (University of California)
Bruce Draine (Princeton University)
M. G. Wolfire (University of Maryland)
Lee Armus (Spitzer Science Center)
Mederic Boquien (Universidad de Antofagasta)
B. Brandl (Astrodynamics & Space Missions, Universiteit Leiden)
Daniel A. Dale (University of Wyoming)
Maud Galametz (ENS-PSL Research University & CNRS, European Southern Observatory)
L. K. Hunt (INAF-Osservatorio Astrofisico di Arcetri)
R. C. Kennicutt (University of Cambridge)
K. Kreckel (Max-Planck-Institut für Astronomie)
D. Rigopoulou (University of Oxford)
p van der werf (Universiteit Leiden)
C Wilson (McMaster University)
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Abstract
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.