The character of the warm ionised medium

TL;DR

This paper characterizes the dense, hot, and localized warm ionised medium in the inner Galactic plane using Herschel observations, revealing its significant contribution to dispersion measures and its potential role in radio-wave scattering phenomena.

Contribution

It introduces a new model of the warm ionised medium as dense, hot, and structured in small-scale clusters around stars, based on combined Herschel and other gas tracers.

Findings

Dense plasma is concentrated in a 130 pc thick disk.

Plasma temperature is approximately 19,000 K.

The plasma significantly affects pulsar dispersion measures.

Abstract

$\textit{Herschel}$ observations of far infrared N$^+$ emission lines have demonstrated that dense plasma, with $n_e\sim30\,{\rm cm^{-3}}$, is ubiquitous in the inner Galactic plane. By combining the information from $\textit{Herschel}$ with other tracers of ionised gas, we build a picture of this dense plasma. We adopt a collisional ionisation model, so the analysis is not tied to a specific energisation mechanism. We find that the dense plasma is concentrated in a disk that is $130\;{\rm pc}$ thick, and makes a significant contribution to radio pulsar dispersion measures in the inner Galactic plane. The strength of the far infrared N$^+$ emission requires high temperatures in the plasma, with $T \simeq 19{,}000\,{\rm K}$ indicated both by the ratio of N$^+$ to C$^+$, and by the ratio of N$^+$ to microwave bremsstrahlung in the inner Galactic plane. This parallels the situation at high Galactic latitudes, where strong optical emission is observed from N$^+$ (and S$^+$), relative to both H$\alpha$ and microwave bremsstrahlung, and suggests a common origin. If so, the same gas provides a natural explanation for the extreme radio-wave scattering phenomena that are sometimes observed in pulsars and quasars. We therefore propose a new picture of the warm ionised medium as seen in emission, in which the plasma is dense, hot, and localised in numerous structures of size $\sim 10^2\,{\rm AU}$ that are clustered around stars.