# Electron Densities and Nitrogen Abundances in Ionized Gas Derived Using [NII] Fine-structure and Hydrogen Recombination lines

## TL;DR

This paper introduces a new method to determine electron densities and nitrogen abundances in ionized galactic gas using [NII] and hydrogen recombination lines, providing insights into galactic chemical distribution.

## Contribution

The study develops a novel approach combining [NII] 205um and hydrogen RRL observations to measure electron densities and nitrogen abundances across the Galaxy.

## Key findings

- Electron densities range from 6 to 170 cm^-3.
- Nitrogen abundance distribution slope is consistent with outer Galaxy studies.
- Inner Galaxy nitrogen abundance gradient is similar to that in the outer Galaxy.

## Abstract

We present a method for deriving the electron density of ionized gas using the ratio of the intensity of the [NII] 205um line to that of Hydrogen radio recombination lines (RRL). 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 [NII] 205um line with the Herschel/HIFI and SOFIA/GREAT instruments and the radio recombination lines with the Green Bank Telescope and the NASA Deep Space Network Deep Space Station 43 (DSS--43) telescope. We find typical electron densities between 6 to 170 cm^-3, which are consistent with those derived at low spectral resolution using the [NII] 205um/122um line ratio with Herschel/PACS on a larger sample of sight lines in the Galactic plane. By matching the electron densities derived from the [NII] 205um/RRL intensity ratio and the [NII] 122um/205um intensity ratio, we derive the nitrogen fractional abundance for most of the velocity components. We investigate the dependence of the N/H ratio with Galactocentric distance in the inner Galaxy (R_gal<6 kpc, which is inaccessible in optical studies due 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 to 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.