Ionization Profiles of Galactic HII Regions
  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. 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 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
increasing distance from the region centers. As a result, giant HII regions may
have a much larger effect in maintaining the ionization of the interstellar
medium. For six of the eight observed HII regions we find a decrease in the
4He+/H+ abundance ratio with 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.
