Ionization of the diffuse gas in galaxies: Hot low-mass evolved stars at
  work -- proceedings IAUS284
  We revisit the question of the ionization of the diffuse medium in late type
galaxies, by studying NGC 891. The most important challenge for the models
considered so far was the observed increase of [OIII]/Hb, [OII]/Hb, and
[NII]/Ha 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 latitude z we look for the models which
simultaneously fit the observed values of the [OIII]/Hb, [OII]/Hb, and [NII]/Ha
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.
