Galaxy Mergers as a Source of Cosmic Rays, Neutrinos, and Gamma Rays

TL;DR

Galaxy mergers can accelerate cosmic rays to very high energies, producing detectable gamma rays and neutrinos, potentially explaining a significant portion of the observed neutrino background.

Contribution

This study demonstrates that galaxy mergers are a viable source of high-energy cosmic rays, gamma rays, and neutrinos, with specific predictions for their energies and fluxes.

Findings

Cosmic rays can reach energies up to 1 EeV in galaxy mergers.

Merging galaxies can produce gamma-ray counterparts detectable by CTA.

Neutrino emissions from mergers could account for 20-60% of IceCube's background.

Abstract

We investigate the shock acceleration of particles in massive galaxy mergers or collisions, and show that cosmic rays (CRs) can be accelerated up to the second knee energy ~0.1-1 EeV and possibly beyond, with a hard spectral index Gamma ~ 2. Such CRs lose their energy via hadronuclear interactions within a dynamical timescale of the merger shock, producing gamma rays and neutrinos as a by-product. If ~ 10 % of the shock dissipated energy goes into CR acceleration, some local merging galaxies will produce gamma-ray counterparts detectable by CTA. Also, based on the concordance cosmology, where a good fraction of the massive galaxies experience a major merger in a cosmological timescale, the neutrino counterparts can constitute ~ 20-60 % of the isotropic background detected by IceCube.