Interactions between Ultra-High-Energy Particles and Protogalactic Environments

TL;DR

This paper studies how ultra-high-energy cosmic rays interact with early galactic environments, revealing the dominant processes, energy deposition rates, and the influence of magnetic fields on cosmic ray confinement and heating.

Contribution

It provides the first detailed analysis of cosmic ray interactions, energy transport, and magnetic field effects in protogalactic environments.

Findings

Pair-production and photo-pion processes dominate at energies above 10^19 eV.

Magnetic fields can confine cosmic rays within protogalaxies within a Myr.

Cosmic ray-driven heating can surpass stellar radiative emission in protogalaxies.

Abstract

We investigate the interactions of energetic hadronic particles (cosmic ray protons) with photons and baryons in protogalactic environments, where the target photons are supplied by the first generations of stars to form in the galaxy and the cosmological microwave background, while the target baryons are the interstellar and circumgalactic medium. We show that pair-production and photo-pion processes are the dominant interactions at particle energies above $10^{19}\;\! {\rm eV}$, while ${\rm pp}$-interaction pion-production dominates at the lower energies in line with expectations from, for example $\gamma$-ray observations of star-forming galaxies and dense regions of our own galaxy's interstellar medium. We calculate the path lengths for the interaction channels and determine the corresponding rates of energy deposition. We have found that protogalactic magnetic fields and their evolution can significantly affect the energy transport and energy deposition processes of cosmic rays. Within a Myr after the onset of star-formation the magnetic field in a protogalaxy could attain a strength sufficient to confine all but the highest energy particles within the galaxy. This enhances the cosmic ray driven self-heating of the protogalaxy to a rate of around $10^{-24}\;\! {\rm erg} \;\!{\rm cm}^{-3}\;\! {\rm s}^{-1}$ for a galaxy with strong star-forming activity that yields 1 core collapse SN event per year. This heating power exceeds even that due to radiative emission from the protogalaxy's stellar populations. However, in a short window before the protogalaxy is fully magnetised, energetic particles could stream across the galaxy freely, delivering energy into the circumgalactic and intergalactic medium.