Diffuse Ionized Gas in the Milky Way Disk

TL;DR

This study investigates the properties and origins of diffuse ionized gas in the Milky Way's first Galactic quadrant using radio recombination lines, revealing two main velocity components linked to Galactic structures and their relation to infrared emission.

Contribution

It provides new insights into the velocity structure, possible origins, and the relationship between diffuse ionized gas and infrared emission in the Milky Way.

Findings

Two dominant velocity components of DIG identified at ~100km/s and ~45km/s.

The 100km/s component likely associated with W43 and Galactic bar dynamics.

Correlation between PAH emission and RRL emission, with PAHs partially destroyed in intense radiation fields.

Abstract

We analyze the diffuse ionized gas (DIG) in the first Galactic quadrant from l=18deg to 40deg using radio recombination line (RRL) data from the Green Bank Telescope. These data allow us to distinguish DIG emission from HII region emission and thus study the diffuse gas essentially unaffected by confusion from discrete sources. We find that the DIG has two dominant velocity components, one centered around 100km/s associated with the luminous HII region W43, and the other centered around 45km/s not associated with any large HII region. Our analysis suggests that the two velocity components near W43 may be caused by non-circular streaming motions originating near the end of the Galactic bar. At lower Galactic longitudes, the two velocities may instead arise from gas at two distinct distances from the Sun, with the most likely distances being ~6kpc for the 100km/s component and ~12kpc for the 45km/s component. We show that the intensity of diffuse Spitzer GLIMPSE 8.0um emission caused by excitation of polyaromatic hydrocarbons (PAHs) is correlated with both the locations of discrete HII regions and the intensity of the RRL emission from the DIG. This implies that the soft ultra-violet photons responsible for creating the infrared emission have a similar origin as the harder ultra-violet photons required for the RRL emission. The 8.0um emission increases with RRL intensity but flattens out for directions with the most intense RRL emission, suggesting that PAHs are partially destroyed by the energetic radiation field at these locations.