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 μ m 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 log ( P th / k ) = 3.8 – 4.6 [ K cm − 3 ] , a value slightly higher than similar determinations, likely because our observations are biased toward star-forming regions.