Speaker: Kenny Wood, University of St Andrews, School of Physics and Astronomy
Abstract: One of the major questions in studies of widespread diffuse ionized gas in galaxies is how ionizing photons can penetrate from OB stars in midplane regions to ionize gas at large heights above the plane. A three dimensional ISM provides the solution - low density voids allow photons to travel much farther than in a smoothly distributed ISM. I will show new results of photoionization calculations in a 3D Supernovae driven hydro simulation of the ISM. Ionizing photons can easily reach to large distances from the midplane. However, the resulting emission measure distributions are broader than observed in the Milky Way and other galaxies. A possible reason for this is the absence of magnetic fields in the dynamical simulations. Our simulations also place limits on the survivability of neutral clouds in the Galactic halo exposed to the ionizing radiation percolating through the models. I will also summarize recent Spitzer observations of gas many kiloparsecs from the midplane of the edge-on galaxy NGC891 that indicate an additional population of very hot ionizing sources are required to explain the observed emission line ratios.