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We revisit the question of the ionization of the diffuse medium in late typegalaxies, by studying NGC 891. The most important challenge for the modelsconsidered so far was the observed increase of [OIII]/Hb, [OII]/Hb, and[NII]/Ha with increasing distance to the galactic plane. We propose a scenariobased on the expected population of massive OB stars and hot low-mass evolvedstars (HOLMES) in this galaxy to explain this observational fact. In theframework of this scenario we construct a finely meshed grid of photoionizationmodels. For each value of the galactic latitude z we look for the models whichsimultaneously fit the observed values of the [OIII]/Hb, [OII]/Hb, and [NII]/Haratios. For each value of z we find a range of solutions which depends on thevalue of the oxygen abundance. The models which fit the observations indicate asystematic decrease of the electron density with increasing z. They becomedominated by the HOLMES with increasing z only when restricting to solar oxygenabundance models, which argues that the metallicity above the galactic planeshould be close to solar. They also indicate that N/O increases with increasingz.

Ionization of the diffuse gas in galaxies: Hot low-mass evolved stars at work -- proceedings IAUS284