Electron Densities and Nitrogen Abundances in Ionized Gas Derived Using [N ii] Fine-structure and Hydrogen Recombination Lines

• Published in: The Astrophysical journal Vol. 886; no. 1; pp. 1 - 16
• Main Authors: Pineda, Jorge L., Horiuchi, Shinji, Anderson, Loren D., Luisi, Matteo, Langer, William D., Goldsmith, Paul F., Kuiper, Thomas B. H., Bryden, Geoff, Soriano, Melissa, W. Lazio, T. Joseph
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 20-11-2019; IOP Publishing
• Subjects: Astronomical instruments; Astrophysics; Deep Space Network; Electron density; Galaxies; Galaxy distribution; Hydrogen; Hydrogen recombinations; ISM: abundances; ISM: clouds; Line of sight; local interstellar matter; Nitrogen; Plasma; regions; Space stations; Space telescopes; Spacecraft components; Spectral resolution; Velocity
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab46c2

We present a method for deriving the electron density of ionized gas using the ratio of the intensity of the [N ii] 205 m line to that of hydrogen radio recombination lines (RRLs). We use this method to derive electron densities of 21 velocity components in 11 lines of sight through the Galaxy, including the Galactic center. We observed, at high spectral resolution, the [N ii] 205 m with the Herschel/HIFI and SOFIA/GREAT instruments and the RRLs with the Green Bank Telescope and the NASA Deep Space Network Deep Space Station 43 (DSS-43) telescope. We find typical electron densities between 8 and 170 cm−3, which are consistent with those derived at low spectral resolution using the [N ii] 205 m/122 m ratio with Herschel/PACS on a larger sample of sight lines in the Galactic plane. By matching the electron densities derived from the [N ii] 205 m/RRL intensity ratio and the [N ii] 122 m/205 m intensity ratio, we derive the nitrogen fractional abundance for most of the velocity components. We investigate the dependence of the N/H ratio on galactocentric distance in the inner Galaxy (Rgal < 6 kpc), which is inaccessible in optical studies owing to dust extinction. We find that the distribution of nitrogen abundances in the inner Galaxy derived from our data has a slope that is consistent with that found in the outer Galaxy in optical studies. This result is inconsistent with some suggestions of a flatter distribution of the nitrogen abundance in the inner Galaxy.