[C II] and [N II] from dense ionized regions in the Galaxy
  The interstellar medium (ISM) consists of highly ionized and neutral atomic,
as well as molecular, components. Knowledge of their distribution is important
for tracing the structure and lifecycle of the ISM. Here we determine the
properties of the highly ionized and neutral weakly ionized gas in the Galaxy
traced by the fine-structure lines of ionized nitrogen, [N II], and ionized
carbon, [C II]. To analyze the ionized ISM we utilize [C II] 158 micron and [N
II] 205 micron lines taken with the high spectral resolution Heterodyne
Instrument in the Far-Infrared (HIFI) on the Herschel Space Observatory along
ten lines of sight towards the inner Galaxy. [N II] emission can be used to
estimate the contribution of the highly ionized gas to the [C II] emission and
separate the highly ionized and weakly ionized neutral gas. We find that [N II]
has strong emission in distinct spectral features along all lines of sight
associated with strong [C II] emission. The [N II] arises from moderate density
extended HII regions or ionized boundary layers of clouds. Comparison of the [N
II] and [C II] spectra in 31 separate kinematic features shows that many of the
[C II] spectra are affected by absorption from low excitation gas associated
with molecular clouds, sometimes strongly so. The apparent fraction of the [C
II] associated with the [N II] gas is unrealistically large in many cases, most
likely due to the reduction of [C II] by absorption. In a few cases the
foreground absorption can be modeled to determine the true source intensity. In
these sources we find that the foreground absorbing gas layer has C$^+$ column
densities of order 10$^{18}$ cm$^{-2}$. [C II] emission arising from strong
sources of [N II] emission is frequently absorbed by low excitation foreground
gas complicating the interpretation of the properties of the ionized and
neutral gas components that give rise to [C II] emission.
