Circum-Galactic Gas and the Isotropic Gamma Ray Background

TL;DR

This paper investigates how interactions between cosmic rays and circum-galactic gas, including hot ionized hydrogen halos around galaxies, significantly contribute to the isotropic gamma-ray background observed by Fermi.

Contribution

It introduces the role of circum-galactic gas, especially hot ionized hydrogen halos, as a notable source of gamma rays contributing to the isotropic background.

Findings

Cosmic ray collisions with circum-galactic gas account for about 10% of the gamma-ray background above 1 GeV.

Extended HII halos of other galaxies also contribute significantly to the gamma-ray background.

The presence of hot ionized gas halos influences the interpretation of gamma-ray observations.

Abstract

Interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way provide the majority of the gamma rays observed by the Fermi Gamma Ray Space Telescope. In addition to the gas which is densely concentrated along the Galactic Disk, hydrodynamical simulations and observational evidence favor the presence of a halo of hot (T~10^6 K) ionized hydrogen (H_II), extending with non-negligible densities out to the virial radius of the Milky Way. We show that cosmic ray collisions with this circum-galactic gas should be expected to provide a significant flux of gamma rays, on the order of 10% of the observed isotopic gamma ray background at energies above 1 GeV. In addition, gamma rays originating from the extended H_II halos of other galaxies along a given line-of-sight should contribute to this background at a similar level.