Data analysis

We derived the H luminosity function (LF) of the H II regions in each galaxy (for details see Rozas et al. 1996) and measured, separately, the diffuse H flux for the whole disc, integrating this out to a radius close to R. We corrected this for unresolved H II regions by extrapolating the statistically incomplete LF below its completeness limit at 10 erg s down to 10 erg s, using the only complete LF available, that of M31 by Walterbos and Braun (1992). To compute the Lyc flux escaping from the H II regions and available to ionize the diffuse gas we extrapolated the LF, using the slope measured below L = 10 erg s, to higher values, and subtracted off the observed LF. The procedure is illustrated in figure 1. A brief explanation of the underlying theory is given in Beckman et al. (these proceedings). A comparison of the available Lyc, and that required to produce the observed diffuse H is shown in table 1. We can see that there is more than sufficient ionizing flux available in all four galaxies.

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