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 radiati on through evolving superbubbles within a smoothly varying H I distribution , we estimate the fraction of ionizing photons emitted by OB associations t hat escapes the H I disk of our Galaxy into the halo and intergalactic medi um (IGM). We consider both coeval star formation and a Gaussian star format ion history with a time spread sigma(t) = 2 Myr. We consider both a uniform H I distribution and a two-phase (cloud/intercloud) model, with a negligib le filling factor of hot gas. We find that the shells of the expanding supe rbubbles quickly trap or attenuate the ionizing flux, so that most of the e scaping radiation escapes shortly after the formation of the superbubble. S uperbubbles of large associations can blow out of the H I disk and form dyn amic chimneys, which allow the ionizing radiation to escape the H I disk di rectly. However, blowout occurs when the ionizing photon luminosity has dro pped well below the association's maximum luminosity. For the coeval star f ormation history, the total fraction of Lyman Continuum photons that escape both sides of the disk in the solar vicinity is [f(esc)] approximate to 0. 15 +/- 0.05. For the Gaussian star formation history, [f(esc)] approximate to 0.06 +/- 0.03, a value roughly a factor of 2 lower than the results of D ove & Shull, where superbubbles were not considered. For a local production rate of ionizing photons Psi(LyC) = 4.95 x 10(7) cm(-2) s(-1), the flux es caping the disk is Phi(LyC) approximate to (1.5 - 3.0) x 10(6) cm(-2) s(-1) for coeval and Gaussian star formation, comparable to the flux required to sustain the Reynolds layer. Rayleigh-Taylor instabilities exist early in t he OB association's evolutionary stages, possibly causing the shell to frag ment and increasing [f(esc)]. However, if a significant fraction of H I is distributed in cold clouds with n(H) similar to 30 cm(-3), [f(esc)] can be reduced by a factor of similar to 2-5 if the cloud properties are similar t o "standard clouds" with a disk geometry.