THE GALACTIC HALO IONIZING FIELD

There has been much debate in recent decades as to what fraction of io nising photons from star-forming regions in the Galactic disk escape i nto the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4 m and the AAT 3.9 m telescopes may now pr ovide the strongest evidence that at least some of the radiation escap es the disk completely. We present a simple model to demonstrate that, while the distance to the Magellanic Stream is uncertain, the observe d emission measures (epsilon(m) approximate to 0.5-1 cm(-6) pc) are mo st plausibly explained cm by photoionisation due to hot, young stars. This model requires that the mean Lyman-limit opacity perpendicular to the disk is tau(LL) approximate to 3, and the covering fraction of th e resolved clouds is close to unity. Alternative sources (e.g. shock, halo, LMC or metagalactic radiation) contribute negligible ionising fl ux.