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Paper: Testing Photoionization Models in the Large Magellanic Cloud and M33
Volume: 331, Extra-Planar Gas
Page: 225
Authors: Voges, E.S.; Walterbos, R.A.M.; Hoopes, C.G.; Oey, M.S.
Abstract: We test photoionization models for diffuse ionized gas (DIG) in two complementary ways. In our first test we study the stellar census of OB stars in 39 HII regions in the Large Magellanic Cloud (LMC) to measure the possible excess amount of Lyman continuum photons available to ionize the DIG. Using photometry only, we find that the average ratio of the predicted to observed ionizing fluxes is 1.4, indicating that stars within HII regions may be a viable ionization source for the DIG. In our second test we use high-resolution spectroscopy of M33 to examine various line ratios, including that of He I/Hα with the purpose of constraining the spectral type of the ionizing stars. We have detected He I in a bright DIG region (EM ~100) indicating that an O7 or earlier star contributes to this region's spectrum. Additionally, we find an increase in the ratios of [NII]/Hα and [SII]/Hα in the DIG as emission measures decrease. The results of both projects support the idea that OB stars are the ionization source for the DIG and begin to constrain their spectral type.
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