The character of the warm ionized medium

Abstract

Herschel observations of far-infrared N⁺ emission lines have demonstrated that dense plasma, with n_ e∼ 30 cm^{-3}, is ubiquitous in the inner Galactic plane. By combining the information from Herschel with other tracers of ionized gas, we build a picture of this dense plasma. We adopt a collisional ionization model, so the analysis is not tied to a specific energization mechanism. We find that the dense plasma is concentrated in a disc that is 130 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 ∼eq 19 000 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 α 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 ionized medium as seen in emission, in which the plasma is dense, hot, and localized in numerous structures of size {∼ } 10^2 au that are clustered around stars.