Cosmic rays, γ-rays, and neutrinos in the starburst nuclei of Arp 220

Abstract

The cores of Arp 220, the closest ultraluminous infrared starburst galaxy, provide an opportunity to study interactions of cosmic rays under extreme conditions. In this paper, we model the populations of cosmic rays produced by supernovae in the central molecular zones of both starburst nuclei. We find that ∼65–100 per cent of cosmic rays are absorbed in these regions due to their huge molecular gas contents, and thus, the nuclei of Arp 220 nearly complete proton calorimeters. As the cosmic ray protons collide with the interstellar medium, they produce secondary electrons that are also contained within the system and radiate synchrotron emission. Using results from χ2 tests between the model and the observed radio spectral energy distribution, we predict the emergent γ-ray and high-energy neutrino spectra and find the magnetic field to be at milligauss levels. Because of the extremely intense far-infrared radiation fields, the γ-ray spectrum steepens significantly at TeV energies due to γ–γ absorption.