On the nature of diffuse ionized gas in galaxies -- I The contribution
  of dust scattering to diffuse line emission
  In this work, we investigate the contribution of dust scattering to the
diffuse H-alpha emission observed in nearby galaxies. As initial conditions for
the spatial distribution of HII regions, gas, and dust, we take three Milky
Way-like galaxies from state-of-the-art cosmological hydrodynamical simulations
that implement different prescriptions for star formation, feedback, and
chemical enrichment. Radiative transfer has been solved a posteriori, using the
publicly-available Monte Carlo code Sunrise to take into account dust
absorption and scattering of the H-alpha photons, originating exclusively from
the HII regions. No contribution from recombinations in the diffuse ionized gas
(DIG) component is explicitly or implicitly included in our model. Our main
result is that the flux arising from scattered light is of the order of 1-2 per
cent of the H-alpha flux coming directly from the HII regions. Building upon
previous studies, we conclude that the DIG contributes lass than 50 per cent of
the total H-alpha emission.
