Diffuse Ionized Gas in the Milky Way Disk

Abstract

We analyze the diffuse ionized gas (DIG) in the first Galactic quadrant from ℓ = 18 ° to 40° using radio recombination line (RRL) data from the Green Bank Telescope. These data allow us to distinguish DIG emission from H ii 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 100 km s − 1 associated with the luminous H ii region W43, and the other centered around 45 km s − 1 not associated with any large H ii region. Our analysis suggests that the two velocity components near W43 may be caused by noncircular 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 ∼6 kpc for the 100 km s − 1 component and ∼12 kpc for the 45 km s − 1 component. We show that the intensity of diffuse Spitzer GLIMPSE 8.0 μ m emission caused by excitation of polyaromatic hydrocarbons (PAHs) is correlated with both the locations of discrete H ii regions and the intensity of the RRL emission from the DIG. This implies that the soft ultraviolet photons responsible for creating the infrared emission have a similar origin as the harder ultraviolet photons required for the RRL emission. The 8.0 μ m 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.