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