The Escape of Ionizing Photons from Ob Associations in Disk Galaxies: Radiation Transfer through Superbubbles

By solving the time-dependent radiation transfer problem of stellar radiation through evolving superbubbles within a smoothly varying H I distribution, we have estimated the fraction of ionizing photons emitted by OB associations that escapes the H I disk of our Galaxy. We considered a coeval star-formation history and a Gaussian star-formation history with a time spread σt = 2 Myr. We find that the shells of the expanding superbubbles quickly trap or attenuate the ionizing flux, such that most of the escaping radiation escapes shortly after the formation of the superbubble. Superbubbles of large associations can blowout of the H I disk and form dynamic chimneys, which allow the ionizing radiation directly to escape the H I disk. However, blowout occurs when the ionizing photon luminosity has dropped well below the association’s maximum luminosity. For the coeval star-formation history, the fraction of photons that escape each side of the disk in the solar vicinity is 〈fesc〉 ≈ 6% (the total fraction of escaping radiation is 2〈fesc〉). For the Gaussian star formation history, 〈fesc〉 ≈ 3%, a value roughly a factor of two lower than the results of Dove & Shull (1994b), where superbubbles were not considered. Assuming a production rate of ionizing photons within 2.5 kpc of ΨLyc = 3.3×107 cm s, the corresponding flux of ionizing photons escaping the disk is ΦLyc ≈ (1 − 2) × 10 cm s for coeval and Gaussian starformation histories, respectively. For the Gaussian star-formation history, the escaping flux is roughly a factor of two smaller than the flux required to keep the Reynolds layer ionized. However, Rayleigh-Taylor and gravitational instabilities exist early in the OB association’s evolutionary stages, possibly causing the shell to fragment and allowing a higher fraction of ionizing radiation to escape the H I disk. Subject headings: H II regions — interstellar medium: diffuse ionized gas — radiation transfer: photoionization —