Cosmic-ray electron population in an extended Galactic or circumgalactic halo

Determine whether cosmic-ray electrons can populate an extended Galactic or circumgalactic halo sufficiently to produce the near-isotropic synchrotron emission represented by the fitted monopole, accounting for inverse-Compton losses and possible continuous supply or reacceleration.

Background

The analysis finds a residual 408 MHz synchrotron monopole of approximately 4–7 K that is not explained by the fitted large-scale disk or halo components. The authors discuss both a nearby Local Bubble contribution and a much larger-scale Galactic or circumgalactic origin.

An extended halo could appear nearly isotropic to an observer within the Galaxy, but its viability depends on whether cosmic-ray electrons can reach and remain in such a region. Inverse-Compton scattering on the cosmic microwave background may prevent electrons from propagating to large distances unless they are continuously supplied or reaccelerated, leaving the physical origin of the monopole unresolved.