Cooling hot plasma and the diffuse ionized gas

The ionization mechanism of the diffuse ionized gas (DIG) in the Milky Way and in other galaxies is still under debate. Various models have been constructed, but up to now none of them can satisfy all the observational constraints. As an alternative scenario, we investigate the photoionization of the DIG by the emission of hot superbubbles. As these are generally accepted to connect the disk with the halo due to their expansion to heights of order 1 kpc, they can reasonably irradiate the surrounding gas high above the galactic disk. Here we compare observed NII/H alpha and SII/H alpha lines of the DIG with those from our models in the vicinity of the bubble and find striking agreement for low superbubble temperatures in the range of (1-3) x 10(5)K and typical electron densities in the range of about 10(-3) - 10(-2) cm(-3) We discuss the issues and further implications for the structure of superbubbles and the DIG on this basis.