The Galactic Halo Ionizing Field

Abstract

There has been much debate in recent decades as to what fraction of ionizing photons from star forming regions in the Galactic disk escape into the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4m and the AAT 3.9m telescopes may now provide the strongest evidence that at least some of the radiation escapes the disk completely. We present a simple model to demonstrate that, while the distance to the Magellanic Stream is uncertain, the observed emission measures (${\cal E}_m \approx 0.5 - 1$ cm$^{-6}$ pc) are most plausibly explained by photoionization due to hot, young stars. This model requires that the mean Lyman-limit opacity perpendicular to the disk is $\tauLL \approx 3$, and the covering fraction of the resolved clouds is close to unity. Alternative sources (e.g. shock, halo, LMC or metagalactic radiation) contribute negligible ionizing flux.