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The fate of ionizing radiation in starburst galaxies is key to understandingcosmic reionization. However, the galactic parameters on which the escapefraction of ionizing radiation depend are not well understood.Ionization-parameter mapping provides a simple, yet effective, way to study theradiative transfer in starburst galaxies. We obtain emission-line ratio maps of[SIII]/[SII] for six, nearby, dwarf starbursts: NGC 178, NGC 1482, NGC 1705,NGC 3125, NGC 7126, and He 2-10. The narrow-band images are obtained with theMaryland-Magellan Tunable Filter at Las Campanas Observatory. Using these data,we previously reported the discovery of an optically thin ionization cone inNGC 5253, and here we also discover a similar ionization cone in NGC 3125. Thislatter cone has an opening angle of 40+/-5 degrees (0.4 ster), indicating thatthe passageways through which ionizing radiation may travel correspond to asmall solid angle. Additionally, there are three sample galaxies that havewinds and/or superbubble activity, which should be conducive to escapingradiation, yet they are optically thick. These results support the scenariothat an orientation bias limits our ability to directly detect escaping Lymancontinuum in many starburst galaxies. A comparison of the star-formationproperties and histories of the optically thin and thick galaxies is consistentwith the model that high escape fractions are limited to galaxies that are oldenough (> 3 Myr) for mechanical feedback to have cleared optically thinpassageways in the ISM, but young enough (< 5 Myr) that the ionizing stars arestill present.

New Constraints on the Escape of Ionizing Photons From Starburst Galaxies Using Ionization-Parameter Mapping