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Ionization by Massive Young Clusters as Revealed by Ionization-Parameter Mapping

Ionization-parameter mapping (IPM) is a powerful technique for tracing the optical depth of Lyman continuum radiation from massive stars. Using narrow-band line-ratio maps, we examine trends in radiative feedback from ordinary HII regions of the Magellanic Clouds and nearby starburst galaxies. We find that the aggregate escape fraction for the Lyman continuum is sufficient to ionize the diffuse, warm ionized medium in the Magellanic Clouds, and that more luminous nebulae are more likely to be optically thin. We apply ionization-parameter mapping to entire starburst galaxies, revealing ionization cones in two nearby starbursts. Within the limits of our small sample, we examine the conditions for the propagation of ionizing radiation beyond the host galaxies.