The origin of the ionization of the diffuse ionized gas in spirals

Abstract

In this paper we make a quantitative study of the hypothesis that the diffuse Hα emitted from the discs of spiral galaxies owes its origin to the ionizing pho tons escaping from H ii regions. The basis of the models is the assumption that a fraction of the Lyman con ti nuum (Lyc) luminosity from the OB stars within each H ii region escapes from the region, leaking into the diffuse gas. A basic input element of any such model is a position and luminosity catalogue in Hα of the H ii regions in the galaxy under examination, down to a low limiting luminosity, and we have previously produced a catalogue of this type for NGC 157. An initial family of models can then be ge ne ra ted in which the Lyc escaping from an H ii region is parametrized in terms of the observed Hα luminosity of the region and the escaping fluxes allowed through the di ffu se disc gas. These models can then be refined using a measured map of H i surface density to effect the down-conversion of the Lyc to Hα. For NGC 157 an H i map was available. Although its mo de ra te angular resolution did limit the accuracy with which we could test our models, the predicted diffuse Hα surface brightness distributions from our mo dels were compared with the observed distributions showing that, in general terms, the hypothesis of density bounding for the H ii regions allows us to predict well the spatial distribution of the diffuse ionized gas. In the model yielding the best fit to the data, the regions of lower luminosity lose a constant fraction of their ionizing flux to their surroundings, while for H ii region luminosities above a specific transition va lue the ionizing escape fraction is a rising function of the Hα luminosity.