Numerical insights into the origin and nature of the ionized interstellar medium

This thesis investigates the origin and nature of diffuse ionized gas (DIG) in the Milky Way and other similar galaxies. I have been developing the 3D radiation-hydrodynamics code, CMacIonize, to include the effect of supernova feedback, an IMF based star formation algorithm, non-equilibrium heating and cooling, and a time-dependent calculation of ionization state. The time-dependent ionization calculation was found to be crucial in reliably simulating the high altitude DIG, with recombination timescales in the low-density gas often exceeding 10~Myr, longer than the time variability of the available ionizing luminosity. The time-dependent ionization state calculations are extended to include various ionized species of helium, carbon, nitrogen, oxygen, neon and sulphur, necessary for simulating collisionally excited emission from the DIG. Synthetic observations of edge on emission line ratios are found to match well to high-altitude observations from Milky Way analogue NGC 891, and inspire novel explanations for hardened line ratios which don't rely on secondary populations of spectrally hard sources. I perform static photoionization simulations through observationally derived 3D density structures using known O stars as the source of ionizing luminosity. I generate a synthetic Hα sky including the effects of dust scattering and absorption. The synthetic sky bears excellent morphological resemblance to the observed Hα sky from surveys such as the Wisconsin H-Alpha Mapper (WHAM). Synthetic skies of other optical and infrared lines are also generated, standing as predictions for future all-sky spectroscopic surveys. Using zoom-in radiation-hydrodynamics simulations, I investigate the effects of the missing temporal dimension in the static simulations. The static simulations assume ionization equilibrium and do not include the effects of supernova feedback. It is found that the equilibrium assumption is likely reliable for this region of higher density material close to the midplane, however some notable effects of supernova feedback are highlighted in the full time-dependent simulations.