HII Region Ionization of the Interstellar Medium: A Case Study of NGC   7538
  Using data from the Green Bank Telescope, we analyze the radio continuum (free-free) and radio recombination line (RRL) emission of the compact HII region NGC 7538 (Sharpless 158). We detect extended radio continuum and hydrogen RRL emission beyond the photodissociation region (PDR) toward the north and east, but a sharp decrease in emission toward the south and west. This indicates that a non-uniform PDR morphology is affecting the amount of radiation "leaking" through the PDR. The strongest carbon RRL emission is found in the western PDR that appears to be dense. We compute a leaking fraction $f_R = 15 \pm 5$ % of the radio continuum emission measured in the plane of the sky which represents a lower limit when accounting for the three-dimensional geometry of the region. We detect an average $^4\textrm{He}^+/\textrm{H}^+$ abundance ratio by number of $0.088 \pm 0.003$ inside the HII region and a decrease in this ratio with increasing distance from the region beyond the PDR. Using Herschel Space Observatory data, we show that small dust temperature enhancements to the north and east of NGC 7538 coincide with extended radio emission, but that the dust temperature enhancements are mostly contained within a second PDR to the east. Unlike the giant HII region W43, the radiation leaking from NGC 7538 seems to only affect the local ambient medium. This suggests that giant HII regions may have a large effect in maintaining the ionization of the interstellar medium. 
Matteo Luisi, L. D. Anderson, Dana S. Balser, T. M. Bania, Trey V.   Wenger
astro-ph.GA