Characterizing the Multiphase Origin of [C II] Emission in M101 and NGC 6946 with Velocity-resolved Spectroscopy

Abstract

The [C II] fine-structure transition at 158 μm is frequently the brightest far-infrared line in galaxies. Due to its low ionization potential, C⁺ can trace the ionized, atomic, and molecular phases of the ISM. We present velocity-resolved [C II] and [N II] pointed observations from SOFIA/GREAT on ~500 pc scales in the nearby galaxies M101 and NGC 6946 and investigate the multiphase origin of [C II] emission over a range of environments. We show that ionized gas makes a negligible contribution to the [C II] emission in these positions using [N II] observations. We spectrally decompose the [C II] emission into components associated with the molecular and atomic phases using existing CO (2-1) and H I data and show that a peak signal-to-noise ratio of 10-15 is necessary for a reliable decomposition. In general, we find that in our pointings ≳50% of the [C II] emission arises from the atomic phase, with no strong dependence on star formation rate, metallicity, or galactocentric radius. We do find a difference between pointings in these two galaxies, where locations in NGC 6946 tend to have larger fractions of [C II] emission associated with the molecular phase than in M101. We also find a weak but consistent trend for fainter [C II] emission to exhibit a larger contribution from the atomic medium. We compute the thermal pressure of the cold neutral medium through the [C II] cooling function and find $\mathrm{log}({P}_{\mathrm{th}}/k)=3.8\mbox{--}4.6\,\,[{\rm{K}}\ \,{\mathrm{cm}}^{-3}]$ , a value slightly higher than similar determinations, likely because our observations are biased toward star-forming regions.