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

TL;DR

This study models the time-dependent ionization of metals in diffuse ionized gas of star-forming galaxies, explaining persistent high-ionization emission lines observed at high altitudes, unlike equilibrium models.

Contribution

It introduces time-dependent metal ionization simulations that reproduce observed emission line ratios and ion persistence in the diffuse ionized gas of galaxies.

Findings

Persistent high-ionization ions like O III and Ne III are explained by long recombination times.

Simulations match observed [Ne III]/[Ne II] ratios in NGC 891.

Equilibrium models fail to reproduce high-altitude ionization states.

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 midplane 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.