THE STAR-FORMING GALAXY CONTRIBUTION TO THE COSMIC MeV AND GeV GAMMA-RAY BACKGROUND

Abstract

While star-forming galaxies could be major contributors to the cosmic GeV\n$\\gamma$-ray background, they are expected to be MeV-dim because of the "pion\nbump" falling off below ~100 MeV. However, there are very few observations of\ngalaxies in the MeV range, and other emission processes could be present. We\ninvestigate the MeV background from star-forming galaxies by running one-zone\nmodels of cosmic ray populations, including Inverse Compton and bremsstrahlung,\nas well as nuclear lines (including $^{26}$Al), emission from core-collapse\nsupernovae, and positron annihilation emission, in addition to the pionic\nemission. We use the Milky Way and M82 as templates of normal and starburst\ngalaxies, and compare our models to radio and GeV--TeV $\\gamma$-ray data. We\nfind that (1) higher gas densities in high-z normal galaxies lead to a strong\npion bump, (2) starbursts may have significant MeV emission if their magnetic\nfield strengths are low, and (3) cascades can contribute to the MeV emission of\nstarbursts if they emit mainly hadronic $\\gamma$-rays. Our fiducial model\npredicts that most of the unresolved GeV background is from star-forming\ngalaxies, but this prediction is uncertain by an order of magnitude. About ~2%\nof the claimed 1 MeV background is diffuse emission from star-forming galaxies;\nwe place a firm upper limit of <~10% based on the spectral shape of the\nbackground. The star-formation contribution is constrained to be small, because\nits spectrum is peaked, while the observed background is steeply falling with\nenergy through the MeV-GeV range.\n