Introduction

The question of the cause of the ionization of the diffuse medium in disc galaxies can be separated into two: the origin of the ionizing photons, and the transparency of the medium between their site of origin and the diffuse medium which they ionize. Here we address essentially the former question, although the two are interlinked, as we will see. There is little doubt that the collective OB star population of a disc galaxy puts out ionizing photons at a rate commensurate with the ionization of its diffuse medium. Typical integrated values of order a few times 10 erg s( 10 photons s) have been measured for the total H flux from spirals (Ferguson et al., 1996), of which some 20% to 30% comes from the diffuse gas between H II regions. Thus in order to support the hypothesis that no ionizing source other than OB stars is needed we need to show that sufficient Lyman continuum (Lyc) is leaking out of the H II regions, which is the topic of the present paper.

© Copyright Astronomical Society of Australia 1997