Ionization of the diffuse gas in galaxies: hot low-mass evolved stars at work

We revisit the question of the ionization of the diffuse medium in late-type galaxies, by studying NGC 891, the prototype of edge-on spiral galaxies. The most important challenge for the models considered so far was the observed increase of [O iii]/Hβ, [O ii]/Hβ and [N ii]/Hα with increasing distance to the galactic plane. We propose a scenario based on the expected population of massive OB stars and hot low-mass evolved stars (HOLMES) in this galaxy to explain this observational fact. In the framework of this scenario we construct a finely meshed grid of photoionization models. For each value of the galactic altitude z we look for the models which simultaneously fit the observed values of the [O iii]/Hβ, [O ii]/Hβ and [N ii]/Hα ratios. For each value of z we find a range of solutions which depends on the value of the oxygen abundance. The models which fit the observations indicate a systematic decrease of the electron density with increasing z. They become dominated by the HOLMES with increasing z only when restricting to solar oxygen abundance models, which argues that the metallicity above the galactic plane should be close to solar. They also indicate that N/O increases with increasing z.