Soap and the Interstellar Froth

We investigate, whether the alleged failure of standard photoion-ization codes of the Diffuse Ionized Gas (DIG) is simply caused by geometrical effects and the insufficient treatment of the radiative transfer. Standard pho-toionization models are applicable only to homogeneous and spherical symmetric nebulae with central ionizing stars, whereas the geometry of disk galaxies requires a 3D distribution of ionizing sources in the disk which illuminate the halo. This change in geometry together with a proper radiative transfer model is expected to substantially influence ionization conditions. Therefore, we developed a new and sophisticated 3D Monte Carlo photoionization code, called SOAP (Simulations Of Astrophysical Plasmas), by adapting an existing 1D code for HII-regions (Och, Lucy, & Rosa 1998) such, that it self-consistently models a 3D disk galaxy with a gaseous DIG halo. First results from a simple (dust-free) model with exponentially decreasing gas densities are presented and the predicted ionization structure of disk and halo is discussed. Theoretical line ratios agree well with observed ones, e.g,. for the halo of NGC 891. Moreover, the fraction of ionizing photons leaving the halo of the galaxy is plotted as a function of varying gas densities. This quantity will be of particular importance for forthcoming studies, because rough estimates indicate that about 7% ionizing photons escape from the halo and contribute to the ionization of the IGM. Given the relatively large number density of normal spiral galaxies, OB-stars could have a much stronger impact on the ionization of the IGM than AGN or QSOs.