Results

The WIM has been found to comprise 20-53% of the total H\ luminosity in nearby star-forming galaxies (see Ferguson etal. 1996). In the LMC, Kennicutt etal. (1995) find this fraction to be %. Our estimate of 0 - 51% for the proportion of escaping ionizing radiation is therefore fully consistent with the possibility that these photons are the ionizing source for the WIM. If the nebulae in our LMC sample are typical in their properties and relationship to the ISM, then these results may also be applicable to other galaxies. We note that our median value of 0.74 in the ratio of observed to predicted H luminosities is in close agreement with the results of Hunter & Massey (1990). The data in that study show a median value of 0.7, using the stellar Lyc estimates of Panagia (1973).

We believe that our sample is representative of the range of nebular properties in the LMC. As seen in Table 1, we include objects with a variety of morphologies, including classical, diffuse HII regions, superbubbles, and composite objects with both shell and diffuse components. The sample also spans a range in H\ luminosity, including some of the brightest nebulae in the LMC (with the exception of 30 Doradus); and also objects an order of magnitude, or more, fainter. We do not find any compelling trends in with either morphology or luminosity. We find that the mean ratio is 0.59 for the superbubbles, 0.89 for the diffuse nebulae, and 0.85 and composite objects. Although the superbubbles appear on average to have a somewhat smaller ratio of observed to predicted luminosity, it is apparent from Table 1 that there is a large scatter in the individual values. Given the small numbers of objects available, we therefore refrain from attributing significance to this possible difference.

Although formally the uncertainties yield a range in escaping ionizing radiation of 0 - 51%, we emphasize the existence of several HII\ regions which appear to be convincingly density-bounded. The lower limit to the fraction of escaping photons is therefore somewhat larger than 0, although it is difficult to constrain this more specifically. We also note the existence of objects that appear to be convincingly radiation-bounded.

Thus a direct comparison of H luminosities with observed stellar ionizing rates shows that the fraction of escaping Lyc radiation is consistent with O star ionization of the WIM. However, at present, the large uncertainties, dominated by uncertainties in stellar ionizing fluxes, do not permit us to rule out alternate mechanisms. But we do find a number of HII regions that appear to be significantly density-bounded, while others appear to be radiation-bounded.

© Copyright Astronomical Society of Australia 1997