3D Photoionization models of diffuse ionized gas

We present 3D photoionization simulations of the diffiuse ionized gas (DIG) in the Milky Way to examine the possibility of using [N II]/H alpha line ratios to probe the 3D structure of the DIG. Compared to data from the Perseus arm, smooth density models produce [N II]/H alpha. values that are lower than observed at small distances above the midplane. This is because N is mostly N2+ Close to the midplane ionizing sources, so [N II]/H alpha is small. As we make our density grid more porous, a smaller luminosity is required to ionize the grid. At small distances from the sources N changes from being being mostly N2+ for the smooth model to mostly N+ in a model with a very small DIG filling factor. We find that simulations with a DIG volume filling factor of around 20% give the best match to observations. This filling factor is remarkably close to that derived from traditional analyses of emission and dispersion measures in the DIG. Clearly, there is no limit to the parameter space that could be investigated using 3D photoionization codes. We comment on future directions for using 3D radiation transfer techniques to critically test 3D dynamical models of the ISM.