[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.

11 pages, 6 figures

References in corpus (5)

• A Herschel [CII] Galactic plane survey III: [CII] as a tracer of star formation
• [CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane
• Herschel Galactic plane survey of [NII] fine structure emission
• [C II] emission from galactic nuclei in the presence of X-rays
• Ionized gas at the edge of the Central Molecular Zone

Cited by in corpus (15)

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• Photodissociation and X-Ray Dominated Regions
• The ALPINE-ALMA [CII] survey: Molecular gas budget in the Early Universe as traced by [CII]
• ALMA Reveals Weak [NII] Emission in "Typical" Galaxies and Intense Starbursts at z=5-6
• Far-infrared Herschel SPIRE spectroscopy of lensed starbursts reveals physical conditions of ionised gas
• [CII] 158 μm self-absorption and optical depth effects
• Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO
• Kinematics and properties of the Central Molecular Zone as probed with [C II]
• SOFIA/FIFI-LS Full-disk [CII] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946
• Herschel Spectroscopy of Early Type Galaxies
• Electron Densities and Nitrogen Abundances in Ionized Gas Derived Using [NII] Fine-structure and Hydrogen Recombination lines
• The Dense Warm Ionized Medium in the Inner Galaxy
• Ionized gas in the Scutum spiral arm as traced in [N II] and [C II]
• Warm ISM in the Sgr A Complex. II. The [C/N] abundance ratio traced by [CII] 158 um and [NII] 205 um observations toward the Arched Filaments at the Galactic center
• Herschel and Odin observations of H2O, CO, CH, CH+, and NII in the barred spiral galaxy NGC 1365. Bar-induced activity in the outer and inner circumnuclear tori