Warm gas and ionizing photons in the local group

Several lines of argument suggest that a large fraction of the baryons in t he universe may be in the form of warm (T similar to 10(5)-10(7) K) gas. In particular, loose groups of galaxies may contain substantial reservoirs of such gas. Observations of the cosmic microwave background by COBE place on ly weak constraints on such an intragroup medium within the Local Group. Th e idea of a Local Group corona dates back at least 40 years (Kahn & Woltjer ). Here we show that gas at T similar to (2-3) x 10(6) K (the approximate v irial temperature of the Local Group)-extremely difficult to observe direct ly-can in principle radiate a large enough flux of ionizing photons to prod uce detectable H alpha emission from embedded neutral clouds. However, addi tional constraints on the corona-the most stringent being pulsar dispersion measures toward the Magellanic Clouds, and the timing mass-rule out an int ragroup medium whose ionizing flux dominates over the cosmic background or the major Local Group galaxies. A cosmologically significant coronal gas ma ss could remain invisible to H alpha observations. More massive galaxy grou ps could contain extensive coronae which are important for the baryon mass and produce a strong, local ionizing flux.