The ionizing radiation from massive stars is a major component of the energy budget of individual galaxies and of the cosmos itself. Knowing about the fate of these photons is fundamental to understand both galaxy and cosmic evolution. The escape fraction of Ionizing Lyman continuum radiation from starburst galaxies generally is only a few percent, and appears to be "trapped". In contrast, discrete HII regions in ordinary star-forming galaxies seem to have substantial leakage of ionizing photons having long path lengths. Dr. Oey and her team investigate this contrasting behavior in different scale phenomena in order to improve our understanding of the fate of these energetic photons. Ultimately this can lead to an observationally tested description of the ultra-violet field emerging from different types of galaxies with different star-formation properties. The team uses modern wide-field, narrow-band imaging technology to directly identify optically thin versus optically thick HII regions and to do ionization-parameter mapping. They plan to quantitatively parameterize the relation between nebular optical depth and cooler interstellar phases for the large and small Magellanic Clouds, and will directly measure optical depths for the nebular population of the nearby galaxies M31 and M33, again relating these to the interstellar medium environment. The technique is also applied to nearby starburst galaxies to evaluate the escape of ionizing radiation and related ionization morphology. These results should have broad impact in the astrophysical community, since the fate of Lyman continuum radiation is one of the most important outstanding issues in cosmic evolution. Likewise, the results should contribute to the understanding of the ionization of the diffuse, warm ionized medium in star-forming galaxies. The project has impact on scales ranging from individual stars to the cosmos itself. The work builds directly on the Dr. Oey's comprehensive program on feedback from massive stars. This project will use the Maryland-Magellan Tunable Narrow Band Filter, which was built with NSF support. A graduate student will be trained in the use of tunable filter devices, which are now being commissioned on existing and planned new-generation telescopes. The PI is involved in several outreach activities, including a partnership with the historic University of Michigan Detroit Observatory, which is starting to open its facilities to undergraduate students and the public. The continued involvement by the PI and her team with the University of Michigan Exhibit Museum of Natural History and more recently, the University of Michigan Museum Studies Program will advance science education in the museum environment.
