ON THE GeV AND TeV DETECTIONS OF THE STARBURST GALAXIES M82 AND NGC 253

Abstract

The GeV and TeV emission from M82 and NGC 253 observed by Fermi, HESS, and\nVERITAS constrains the physics of cosmic rays (CRs) in these dense starbursts.\nWe argue that the gamma rays are predominantly hadronic in origin, as expected\nby previous studies. The measured fluxes imply that pionic losses are efficient\nfor CR protons in both galaxies: we show that a fraction F_cal ~ 0.2 - 0.4 of\nthe energy injected in high energy primary CR protons is lost to inelastic\nproton-proton collisions (pion production) before escape, producing gamma rays,\nneutrinos, and secondary electrons and positrons. We discuss the factor ~2\nuncertainties in this estimate, including supernova rate and leptonic\ncontributions to the GeV-TeV emission. We argue that gamma-ray data on ULIRGs\nlike Arp 220 can test whether M82 and NGC 253 are truly calorimetric, and we\npresent upper limits on Arp 220 from the Fermi data. We show that the observed\nratio of the GeV to GHz fluxes of the starbursts suggests that non-synchrotron\ncooling processes are important for cooling the CR electron/positron\npopulation. We briefly reconsider previous predictions in light of the\ngamma-ray detections, including the starburst contribution to the gamma-ray\nbackground and CR energy densities. Finally, as a guide for future studies, we\nlist the brightest star-forming galaxies on the sky and present updated\npredictions for their gamma-ray and neutrino fluxes.\n