Escape of Cosmic Rays from the Galaxy and Effects on the Circumgalactic Medium.

Abstract

The escape of cosmic rays from the Galaxy is expected to shape their spectrum inside the Galaxy. Yet, this phenomenon is very poorly understood and, in the absence of a physical description, it is usually modeled as a free escape from a given boundary, typically located at a few kiloparsec distance from the Galactic disc. We show that the assumption of free escape leads to the conclusion that the cosmic ray current propagating in the circumgalactic medium is responsible for a nonresonant cosmic ray induced instability that in turn leads to the generation of a magnetic field of strength ∼2×10^{-8} G on a scale ∼10 kpc around our Galaxy. The self-generated diffusion produces large gradients in the particle pressure that induce a displacement of the intergalactic medium with velocity ∼10-100 km/s. Cosmic rays are then carried away by advection. If the overdensity of the intergalactic gas in a region of size ∼10 kpc around our Galaxy is ≳100 with respect to the cosmological baryon density Ω_{b}ρ_{cr}, then the flux of high energy neutrinos due to pion production becomes comparable with the flux of astrophysical neutrinos recently measured by IceCube.