[C II] and [N II] from dense ionized regions in the Galaxy

Abstract: 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.