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

Understand

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.