The ionization of diffuse ionized gas in galaxies: A comparison with the Reynolds‐layer in the Milky Way

A thick disk of ionized gas similar to the Reynolds‐layer of the Galaxy has recently been detected in several edge‐on galaxies. We have measured line ratios of diagnostic emission lines such as [NII]/Hα, [SII]/Hα originating in this extraplanar diffuse ionized gas (DIG). In addition, we have compiled from the literature emission line ratios measured for diffuse ionized gas in nearby late‐type spiral, early‐type disk (S0/Sa), and Magellanic‐type irregular galaxies.From this compilation one trend emerges. For most objects [SII] and [NII] lines are both stronger in DIG spectra than in the emission from HII regions. It is only for DIG in Milky Way and irregular galaxies that [SII] lines are strong while [NII] line strengths are comparable to HII regions. The DIG in the Milky Way is therefore not typical for physical conditions in the diffuse ionized gas in external galaxies.The observed spread in excitation conditions allows us to confine possible ionization models. It is concluded that photoionization by a d...