On the Structure and Morphology of the "Diffuse Ionized Medium" in Star-forming Galaxies

Deep Hα images of a sample of nearby late-type spiral galaxies have been analyzed to characterize the morphology and energetic significance of the "diffuse ionized medium" (DIM). We find that the DIM properties can be reasonably unified as a function of relative surface brightness (Σ/, where is the mean Hα surface brightness within regions lying above a fixed, very faint isophotal level). We measured the images down to this common isophotal limit and constructed a fundamental dimensionless surface brightness distribution function that describes the dependence of the area (normalized to total area) occupied by gas with a given relative surface brightness (Σ/). This function determines both the flux and area contribution by the DIM to global values. The function is found to be almost the same at high surface brightness (Σ/≳1) and less similar for Σ/≲1. We show the universal distribution function at high surface brightness can be understood as a consequence of the general properties of H II regions, including their Hα luminosity function and exponential radial brightness profiles. We suggest that relative surface brightness (rather than an absolute value) is a more physically meaningful criterion to discriminate the DIM from H II regions. The use of the dimensionless distribution function to quantify the DIM is consistent with the fundamentally morphological definition of the DIM as being "diffuse." The difference in the distribution function from galaxy to galaxy at low surface brightness quantifies the different prominence of the DIM in the galaxies. This variation is found to be consistent with results from other complementary ways of determining the DIM's global importance. The variation of the DIM among the galaxies that is indicated by the distribution function is small enough to guarantee that the fractional contribution of the DIM to the global Hα luminosity in the galaxies is fairly constant, as has been observed. The continuous transition from H II regions to the DIM in the distribution function suggests that the ionizing energy for the DIM mainly comes from H II regions, consistent with the "leaky H II regions model."