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Cosmic rays (CRs) in starburst galaxies produce high energy gamma-rays bycolliding with the dense interstellar medium (ISM). Arp 220 is the nearestultra luminous infrared galaxy (ULIRG) that has star-formation at extremelevels, 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 MeVfrom 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 showsno significant variability over the observation period and it is consistentwith the quasi-linear scaling relation between the gamma-ray luminosity andtotal infrared luminosity for star-forming galaxies, suggesting that thesegamma-rays arise from CR interactions. As the high density medium of Arp 220makes it an ideal CR calorimeter, the gamma-ray luminosity can be used tomeasure the efficiency of powering CRs by supernova (SN) remnants given a knownsupernova rate in Arp 220. We find that this efficiency is about $4.2\pm2.6\%$for CRs above 1 GeV.

First detection of GeV emission from an ultraluminous infrared galaxy: Arp 220 as seen with the Fermi Large Area Telescope