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

Abstract

Abstract We present a method for deriving the electron density of ionized gas using the ratio of the intensity of the [N ii] 205 μm 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 μm 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 μm/122 μm 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 μm/RRL intensity ratio and the [N ii] 122 μm/205 μm 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 (R gal < 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.