Cosmic Rays, Gamma-Rays, and Neutrinos in the Starburst Nuclei of Arp 220

TL;DR

This study models cosmic ray interactions in Arp 220's starburst nuclei, revealing high absorption rates, strong magnetic fields, and gamma-ray attenuation, providing insights into cosmic ray physics in extreme galactic environments.

Contribution

It presents a detailed model of cosmic ray populations and their secondary emissions in Arp 220, highlighting the nuclei as nearly complete proton calorimeters with implications for gamma-ray and neutrino observations.

Findings

High cosmic ray absorption (~65-100%) in Arp 220 nuclei

Magnetic fields estimated at milligauss levels

Gamma-ray spectrum steepens at TeV energies due to gamma-gamma absorption

Abstract

The cores of Arp 220, the closest ultra-luminous 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% 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 chi-squared tests between the model and the observed radio spectral energy distribution, we predict the emergent gamma-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 gamma-ray spectrum steepens significantly at TeV energies due to gamma-gamma absorption.