# Do Cosmic Rays Heat the Early Intergalactic Medium?

## TL;DR

Cosmic rays likely played a significant role in heating the early intergalactic medium before reionization, with implications for understanding early universe magnetic fields and supernova activity.

## Contribution

This study models the impact of low-energy cosmic rays on the early IGM, highlighting their heating effect and the importance of CR diffusion and self-confinement.

## Key findings

- CRs heat the IGM by 10-200 K by redshift 10.
- CRs contribute negligibly to ionization of the IGM.
- The heating pattern depends on CR diffusion and magnetic field structure.

## Abstract

Cosmic rays (CRs) govern the energetics of present-day galaxies and might have also played a pivotal role during the Epoch of Reionization. In particular, energy deposition by low-energy ($E \lesssim 10$ MeV) CRs accelerated by the first supernovae, might have heated and ionized the neutral intergalactic medium (IGM) well before ($z \approx 20$) it was reionized, significantly adding to the similar effect by X-rays or dark matter annihilations. Using a simple, but physically motivated reionization model, and a thorough implementation of CR energy losses, we show that CRs contribute negligibly to IGM ionization, but heat it substantially, raising its temperature by $\Delta T=10-200$ K by $z=10$, depending on the CR injection spectrum. Whether this IGM pre-heating is uniform or clustered around the first galaxies depends on CR diffusion, in turn governed by the efficiency of self-confinement due to plasma streaming instabilities that we discuss in detail. This aspect is crucial to interpret future HI 21 cm observations which can be used to gain unique information on the strength and structure of early intergalactic magnetic fields, and the efficiency of CR acceleration by the first supernovae.