Interactions between ultra-high-energy particles and protogalactic environments

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 pairproduction and photo-pion processes are the dominant interactions at particle energies above 1019 eV, while proton-proton (pp) interaction pion-production dominates at the lower energies in line with expectations from, for example γ -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 aMyr after the onset of starformation the magnetic field in a protogalaxy could attain a strength sufficient to confine all but the highest energy particleswithin the galaxy. This enhances the cosmic ray-driven self-heating of the protogalaxy to a rate of around 10-24 erg cm-3 s-1 for a galaxy with strong star-forming activity that yields one core collapse supernova 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 magnetized, energetic particles could stream across the galaxy freely, delivering energy into the circumgalactic and intergalactic medium.