Using Green Bank Telescope radio recombination line (RRL) data, we analyze the role of leaking radiation from HII regions in maintaining the ionization of the interstellar medium (ISM). We observed a sample of eight Galactic HII regions of various sizes, morphologies, and luminosities. For each region, the hydrogen RRL intensity decreases roughly as a power-law with the distance from the center of the region. This suggests that radiation leaking from the HII region is responsible for the majority of surrounding ionized gas producing RRL emission. Our results further indicate that the hydrogen RRL intensity appears to be fundamentally related to the HII region sizes traced by their photodissociation regions, such that physically smaller HII regions show a steeper decrease in intensity with an increasing distance from the region centers. As a result, giant HII regions may have a much larger effect in maintaining the ionization of the ISM. For six of the eight observed HII regions, we find a decrease in the ^4^He^+^/H^+^ abundance ratio with an increasing distance, indicating that He-ionizing photons are being absorbed within the ionization front of the HII region. There is enhanced carbon RRL emission toward directions with strong continuum background, suggesting that the carbon emission is amplified by stimulated emission.

Cone search capability for table J/ApJS/241/2/table1 (H II region properties)

Cone search capability for table J/ApJS/241/2/pointings (Observed positions (table 3) and radio continuum temperatures and source properties (table 5))