Time-dependent metal ionization and the persistence of collisionally excited emission lines in the diffuse ionized gas of star-forming galaxies

ABSTRACT We extend our time-dependent hydrogen ionization simulations of diffuse ionized gas to include metals important for collisional cooling and diagnostic emission lines. The combination of heating from supernovae and time-dependent collisional and photoionization from mid-plane OB stars produces emission line intensities (and emission line ratios) that follow the trends observed in the Milky Way and other edge-on galaxies. The long recombination times in low-density gas result in persistent large volumes of ions with high ionization potentials, such as O iii and Ne iii. In particular, the vertically extended layers of Ne iii in our time-dependent simulations result in [Ne iii] 15 $\mu$m/[Ne ii] 12 $\mu$m emission line ratios in agreement with observations of the edge-on galaxy NGC 891. Simulations adopting ionization equilibrium do not allow for the persistence of ions with high ionization states and therefore cannot reproduce the observed emission lines from low-density gas at high altitudes.