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Using Green Bank Telescope radio recombination line (RRL) data, we analyzethe role of leaking radiation from HII regions in maintaining the ionization ofthe interstellar medium. We observed a sample of eight Galactic HII regions ofvarious sizes, morphologies, and luminosities. For each region the hydrogen RRLintensity decreases roughly as a power-law with distance from the center of theregion. This suggests that radiation leaking from the HII region is responsiblefor the majority of surrounding ionized gas producing RRL emission. Our resultsfurther indicate that the hydrogen RRL intensity appears to be fundamentallyrelated to the HII region sizes traced by their photodissociation regions, suchthat physically smaller HII regions show a steeper decrease in intensity withincreasing distance from the region centers. As a result, giant HII regions mayhave a much larger effect in maintaining the ionization of the interstellarmedium. For six of the eight observed HII regions we find a decrease in the4He+/H+ abundance ratio with increasing distance, indicating that He-ionizingphotons are being absorbed within the ionization front of the HII region. Thereis enhanced carbon RRL emission toward directions with strong continuumbackground, suggesting that the carbon emission is amplified by stimulatedemission.

Ionization Profiles of Galactic HII Regions