Star-forming galaxies produce gamma-rays primarily via pion production , resulting from inelastic collisions between cosmic ray protons and the interstellar medium ( ISM ) .
The dense ISM and high star formation rates of luminous and ultra-luminous infrared galaxies ( LIRGs and ULIRGs ) imply that they should be strong gamma-ray emitters , but so far only two LIRGs have been detected .
Theoretical models for their emission depend on the unknown fraction of cosmic ray protons that escape these galaxies before interacting .
We analyze Fermi -LAT data for 82 of the brightest IRAS LIRGs and ULIRGs .
We examine each system individually and carry out a stacking analysis to constrain their gamma-ray fluxes .
We report the detection of the nearest ULIRG Arp 220 ( \sim 4.6 \sigma ) .
We observe a gamma-ray flux ( 0.8–100 GeV ) of 2.4 \times 10 ^ { -10 } phot cm ^ { -2 } s ^ { -1 } with photon index of 2.23 ( 8.2 \times 10 ^ { 41 } ergs s ^ { -1 } at 77 Mpc ) We also derive upper limits for the stacked LIRGs and ULIRGs .
The gamma-ray luminosity of Arp 220 and the stacked upper limits agree with calorimetric predictions for dense star-forming galaxies .
With the detection of Arp 220 , we extend the gamma-ray–IR luminosity correlation to the high luminosity regime with \log { L _ { 0.1 - 100 \textrm { GeV } } } = 1.25 \times \log { L _ { 8 - 1000 \mu \textrm { m } } } +26.7 as well as the gamma-ray–radio continuum luminosity correlation with log { L _ { 0.1 - 100 \textrm { GeV } } } = 1.22 \times \log { L _ { 1.4 \textrm { GHz } } } +13.3 .
The current survey of Fermi -LAT is on the verge of detecting more LIRGs/ULIRGs in the local universe , and we expect even more detections with deeper Fermi -LAT observations or the next generation of gamma-ray detectors .