Herschel Galactic plane survey of [NII] fine structure emission

TL;DR

This paper presents the first large-scale, high-resolution survey of ionized nitrogen in the Galactic Plane, revealing its distribution, electron densities, and correlation with [CII], providing new insights into ionized gas in our galaxy.

Contribution

It introduces a comprehensive survey of [NII] emission at 122 and 205 μm in the Galactic Plane, measuring electron densities and linking [NII] with [CII] emission, which was not previously done at this scale.

Findings

[NII] emission is extended and reliably detected across the Galactic Plane.

Electron densities in the ionized gas range from 10 to 50 cm$^{-3}$, averaging 29 cm$^{-3}$.

Between one-third and one-half of [CII] emission is associated with ionized gas.

Abstract

We present the first large scale high angular resolution survey of ionized nitrogen in the Galactic Plane through emission of its two fine structure transitions ([NII]) at 122 $\mu$m and 205 $\mu$m. The observations were largely obtained with the PACS instrument onboard the Herschel Space Observatory. The lines-of-sight were in the Galactic plane, following those of the Herschel OTKP project GOT C+. Both lines are reliably detected at the 10$^{-8}$ - 10$^{-7}$ $W$m$^{-2}$sr$^{-1}$ level over the range -60$^{o}$ $\leq$ $l$ $\leq$ 60$^{o}$. The $rms$ of the intensity among the 25 PACS spaxels of a given pointing is typically less than one third of the mean intensity, showing that the emission is extended. [NII] is produced in gas in which hydrogen is ionized, and collisional excitation is by electrons. The ratio of the two fine structure transitions provides a direct measurement of the electron density, yielding $n(e)$ largely in the range 10 to 50 cm$^{-3}$ with an average value of 29 cm$^{-3}$ and N$^+$ column densities 10$^{16}$ to 10$^{17}$ cm$^{-2}$. [NII] emission is highly correlated with that of [CII], and we calculate that between 1/3 and 1/2 of the [CII] emission is associated with the ionized gas. The relatively high electron densities indicate that the source of the [NII] emission is not the Warm Ionized Medium (WIM), which has electron densities more than 100 times smaller. Possible origins of the observed [NII] include the ionized surfaces of dense atomic and molecular clouds, the extended low density envelopes of HII regions, and low-filling factor high-density fluctuations of the WIM.