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Constraining Gamma-Ray Emission from Luminous Infrared Galaxies with Fermi-LAT; Tentative Detection of Arp 220

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 (~4.6sigma). We observe a gamma-ray flux (0.8--100 GeV) of 2.4e-10 phot cm^-2 s^-1 with photon index of 2.23 (8.2e10^41 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 GeV]) = 1.25log(L_[8-1000]) + 26.7 as well as the gamma-ray--radio continuum luminosity correlation with log(L_[0.1-100 GeV]) = 1.22log(L_[1.4 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.