The Ultimate Fate of Cosmic Rays from Galaxies and their Role in the Intergalactic Medium

TL;DR

Cosmic rays from galaxies significantly influence the intergalactic medium's pressure and magnetic fields, especially at high redshifts, suggesting a substantial nonthermal component in the IGM's evolution.

Contribution

This paper demonstrates the potential dominance of cosmic ray pressure in the IGM and explores their confinement and magnetic field generation, which are novel insights into cosmic ray impact on cosmic structures.

Findings

CR pressure can rival thermal pressure in the IGM.

CRs may generate magnetic fields >~ 10^{-13} G.

CRs likely reside at least 30 kpc from their sources.

Abstract

The majority of cosmic rays (CRs) generated by star-forming galaxies escape them and enter the intergalactic medium (IGM). Galactic wind termination shocks might also accelerate CRs. I show that the mean pressure of these CRs can reach to within an order of magnitude of the mean Lyman-alpha forest thermal pressure. At z >~ 1, their pressure may have even been dominant. I also demonstrate that, whichever IGM phase the CRs reside in, they contribute significantly to its pressure if its temperature is ~$10^4$ K, as long as pionic and Coulomb losses are negligible. Where CRs end up depends on the structure and strength of intergalactic magnetic fields. I argue that CRs end up at least 30 kpc from their progenitor galaxies. CRs may self-confine in the IGM to the sound speed, generating >~ $10^{-13}$ G magnetic fields. These considerations imply the existence and importance of a nonthermal IGM.