Introduction

Are O stars the principal source of ionization for the diffuse, warm, ionized medium (WIM) in galaxies? Most recent evidence has suggested that this is probably the case. O stars are one of the only sources capable of meeting the power requirement (e.g., Reynolds 1984) and spatial distribution (Miller & Cox 1993; Dove & Shull 1994) of this pervasive component of the interstellar medium (ISM). The localization of diffuse, ionized gas near HII regions is also highly suggestive of ionization by O stars (Walterbos & Braun 1994; Ferguson 1996).

But HII regions have traditionally been considered as essentially radiation-bounded objects, and aggregate samples appear to be broadly consistent with this interpretation (e.g., Kennicutt 1988; Banfi etal. 1993). In that case, there would be essentially no significant ionizing flux escaping the nebulae, therefore requiring one of the alternate mechanisms for ionizing the WIM. We can investigate this question by examining individual HII regions: how do the ionizing fluxes from the observed O stars actually compare with the observed nebular luminosities? Here we summarize this simple study, which is reported in greater detail by Oey & Kennicutt (1997).

© Copyright Astronomical Society of Australia 1997