Warm Gas and Ionizing Photons in the Local Group

Abstract

We investigate how much warm (T \sim 10^6 K) gas may exist in a Local Group corona, and, in particular, whether such a corona could be detected indirectly through its ionizing radiation. We calculate the non-LTE cooling of warm, low-density gas and the resulting ionizing photon emissivities for a range of metallicities. In principle a corona can produce a photon flux which significantly exceeds the cosmic ionizing background. However, the additional constraints which can be imposed on intragroup gas rule out models which produce detectable ionizing photon fluxes at the Milky Way's distance from the Local Group barycenter.