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 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 sigma_t = 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 f_esc approx 6% (the total fraction of escaping radiation is 2\f_esc). For the Gaussian star formation history, f_esc approx 3%, a value roughly a factor of two lower than the results of Dove and Shull (1994), where superbubbles were not considered. 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.Keywords
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