The Metagalactic Ionizing Field in the Local Group

We discuss the sources which are likely to dominate the ionizing field throughout the Local Group. In terms of the limiting flux to produce detectable Hα emission (∼ 4 − 10 × 10 phot cm s), the four dominant galaxies (M31, Galaxy, M33, LMC) have spheres of influence which occupy a small fraction (5− 10%) of the Local Volume. There are at least two possible sources of ionization whose influence could be far more pervasive: (i) a cosmic background of ionizing photons; (ii) a pervasive warm plasma throughout the Local Group. The COBE FIRAS sky temperature measurements permit a wide variety of plasmas with detectable ionizing fields. It has been suggested (Blitz et al. 1996; Spergel et al. 1996; Sembach et al. 1995, 1998) that a substantial fraction of high velocity clouds are external to the Galaxy but within the Local Group. Deep Hα detections are the crucial test of these claims and, indeed, provide a test bed for the putative Local Group corona. 1