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

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, a value slightly higher than similar determinations, likely because our observations are biased toward star-forming regions.