# Gamma-Ray Emission from Arp 220: Indications of an Active Galactic Nucleus

## TL;DR

This study investigates gamma-ray emissions from Arp 220, revealing an excess that suggests the presence of an active galactic nucleus contributing additional cosmic rays beyond star formation explanations.

## Contribution

It provides evidence for an active galactic nucleus in Arp 220 through multi-wavelength diagnostics, highlighting discrepancies between gamma-ray and radio observations.

## Key findings

- Gamma-ray flux exceeds predictions from star formation alone.
- An AGN likely contributes additional cosmic rays in Arp 220.
- Discrepancies suggest the western nucleus hosts an active galactic nucleus.

## Abstract

Extragalactic cosmic ray populations are important diagnostic tools for tracking the distribution of energy in nuclei and for distinguishing between activity powered by star formation versus active galactic nuclei (AGNs). Here, we compare different diagnostics of the cosmic ray populations of the nuclei of Arp 220 based on radio synchrotron observations and the recent gamma-ray detection. We find the gamma-ray and radio emission to be incompatible; a joint solution requires at minimum a factor of 4 - 8 times more energy coming from supernovae and a factor of 40 - 70 more mass in molecular gas than is observed. We conclude that this excess of gamma-ray flux in comparison to all other diagnostics of star-forming activity indicates that there is an AGN present that is providing the extra cosmic rays, likely in the western nucleus.