Galaxy mergers as a source of cosmic rays, neutrinos, and gamma rays

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 of Γ 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 the Cherenkov Telescope Array. 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.