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

Abstract

By solving the time-dependent radiation transfer problem of stellar radiation through evolving superbubbles within a smoothly varying H I distribution, we estimate the fraction of ionizing photons emitted by OB associations that escapes the H I disk of our Galaxy into the halo and intergalactic medium (IGM). We consider both coeval star formation and a Gaussian star formation history with a time spread σt = 2 Myr. We consider both a uniform H I distribution and a two-phase (cloud/intercloud) model, with a negligible filling factor of hot gas. We find that the shells of the expanding superbubbles quickly trap or attenuate the ionizing flux, so that most of the escaping radiation escapes shortly after the formation of the superbubble. Superbubbles of large associations can blow out of the H I disk and form dynamic chimneys, which allow the ionizing radiation to escape the H I disk directly. However, blowout occurs when the ionizing photon luminosity has dropped well below the association's maximum luminosity. For the coeval star formation history, the total fraction of Lyman Continuum photons that escape both sides of the disk in the solar vicinity is 〈fesc〉 ≈0.15 ± 0.05. For the Gaussian star formation history, 〈fesc〉 ≈0.06 ± 0.03, a value roughly a factor of 2 lower than the results of Dove & Shull, where superbubbles were not considered. For a local production rate of ionizing photons ΨLyC = 4.95 × 107 cm-2 s-1, the flux escaping the disk is ΦLyC ≈(1.5 - 3.0) × 106 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 the OB association's evolutionary stages, possibly causing the shell to fragment and increasing 〈esc〉. However, if a significant fraction of H I is distributed in cold clouds with nH ∼ 30 cm-3, 〈esc〉 can be reduced by a factor of ∼2-5 if the cloud properties are similar to "standard clouds" with a disk geometry.