On the Structure and Morphology of the `Diffuse Ionized Medium' in Star-Forming Galaxies

Deep H$\alpha$ 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, by using a new method to quantify the DIM importance in galaxies. This new approach is more consistent with the fundamentally morphological definition of the DIM as being `Diffuse', compared to the traditional way adopted in previous studies that could only isolate the DIM based on an absolute surface brightness criterion. Our results suggest that the variation of the DIM's significance among the galaxies is small enough so that the fractional contribution of the DIM to the global H$\alpha$ luminosity in the galaxies is fairly constant, as has been observed. We found a smooth structural transition from HII regions to the DIM, suggesting that the ionizing energy for the DIM mainly comes from HII regions.