First detection of GeV emission from an ultraluminous infrared galaxy:
  Arp 220 as seen with the Fermi Large Area Telescope
  Cosmic rays (CRs) in starburst galaxies produce high energy gamma-rays by
colliding with the dense interstellar medium (ISM). Arp 220 is the nearest
ultra luminous infrared galaxy (ULIRG) that has star-formation at extreme
levels, so it has long been predicted to emit high-energy gamma-rays. However,
no evidence of gamma-ray emission was found despite intense efforts of search.
Here we report the discovery of high-energy gamma-ray emission above 200 MeV
from Arp 220 at a confidence level of $\sim 6.3 \sigma $ using 7.5 years of
\textsl {Fermi} Large Area Telescope observations. The gamma-ray emission shows
no significant variability over the observation period and it is consistent
with the quasi-linear scaling relation between the gamma-ray luminosity and
total infrared luminosity for star-forming galaxies, suggesting that these
gamma-rays arise from CR interactions. As the high density medium of Arp 220
makes it an ideal CR calorimeter, the gamma-ray luminosity can be used to
measure the efficiency of powering CRs by supernova (SN) remnants given a known
supernova rate in Arp 220. We find that this efficiency is about $4.2\pm2.6\%$
for CRs above 1 GeV.
