The character of the warm ionised medium

Abstract

$\text{Herschel}$ observations of far infrared N${}^{+}$ emission lines have demonstrated that dense plasma, with $n_e\sim 30\mathrm{c}{\mathrm{m}}^{-3}$, is ubiquitous in the inner Galactic plane. By combining the information from $\text{Herschel}$ 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 $130\mathrm{pc}$ 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 $T\simeq 19,000\mathrm{K}$ 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 $\sim 10^2\mathrm{AU}$ that are clustered around stars.

17 pages, 14 figures, Accepted to MNRAS

References in corpus (7)

• The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey II: The First Four Epochs
• Scattering analysis of LOFAR pulsar observations
• The H+ Region Contribution to [C II] 158 Micron Emission
• Constraining Spinning Dust Parameters with the WMAP Five-Year Data
• Extreme radio-wave scattering associated with hot stars
• Identification of Spinning Dust in Halpha-Correlated Microwave Emission
• Heating of the Warm Ionized Medium by Low-Energy Cosmic Rays

Cited by in corpus (5)

• The GBT Diffuse Ionized Gas Survey (GDIGS): Survey Overview and First Data Release
• The C-Band All-Sky Survey (C-BASS): Template Fitting of Diffuse Galactic Microwave Emission in the Northern Sky
• Electron Densities and Nitrogen Abundances in Ionized Gas Derived Using [NII] Fine-structure and Hydrogen Recombination lines
• The Dense Warm Ionized Medium in the Inner Galaxy
• Thermal Pressures in the Interstellar Medium away from Stellar Environments