Diffuse γ-rays from galactic halos

The γ-ray sky as we know it today is a composite of the emission from point sources and diffuse emission (see Fig.1). The most prominent feature is the galactic plane, in which interactions between cosmic rays and the thermal gas lead to γ-ray emission by π-decay and bremsstrahlung. On top of this diffuse emission we see point sources all over the map, part of them being pulsars, another part distant AGN, and also a significant fraction of yet unidentified sources. But we also see that the diffuse emission extends out of the galactic plane up to the poles. There appears to be an isotropic emission component which is presumably extragalactic in origin and may be understood as blend of unresolved AGN, but there is also considerable galactic emission. At higher latitudes interactions between cosmic rays and thermal gas still play a role, but inverse-Compton scattering of ambient photons by cosmic ray electrons becomes increasingly important. This emission tells us about the physical conditions in the halo as seen by the cosmic ray particles, and it may also reveal previously hidden gas, e.g. baryonic dark matter.