CONSTRAINING GAMMA-RAY EMISSION FROM LUMINOUS INFRARED GALAXIES WITH FERMI-LAT; TENTATIVE DETECTION OF ARP 220

Abstract

ABSTRACT 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 Infrared Astronomical Satellite 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 (∼4.6σ). We observe a gamma-ray flux (0.8–100 GeV) of 2.4 × 10−10 phot cm−2 s−1 with a photon index of 2.23 (8.2 × 1041 erg s−1 at 77 Mpc). We also derive upper limits (ULs) for the stacked LIRGs and ULIRGs. The gamma-ray luminosity of Arp 220 and the stacked ULs 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 = 1.25 × log L 8 − 1000 μ m + 26.7 as well as the gamma-ray–radio continuum luminosity correlation with log L 0.1 − 100 GeV = 1.22 × log 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.