Characterizing the Multi-Phase Origin of [CII] Emission in M101 and NGC
  6946 with Velocity Resolved Spectroscopy
  The [CII] fine-structure transition at 158 micron 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 [CII] and [NII] pointed observations from SOFIA/GREAT on ~500 pc
scales in the nearby galaxies M101 and NGC 6946 and investigate the multi-phase
origin of [CII] emission over a range of environments. We show that ionized gas
makes a negligible contribution to the [CII] emission in these positions using
[NII] observations. We spectrally decompose the [CII] emission into components
associated with the molecular and atomic phases using existing CO(2-1) and HI
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 greater than
or equal to 50% of the [CII] 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 [CII] emission
associated with the molecular phase than in M101. We also find a weak but
consistent trend for fainter [CII] emission to exhibit a larger contribution
from the atomic medium. We compute the thermal pressure of the cold neutral
medium through the [CII] 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 towards star-forming regions.
