Brian C. Lacki, Shunsaku Horiuchi, John F. Beacom
[Abridged] While star-forming galaxies could be major contributors to the cosmic GeV gamma-ray background, they are expected to be MeV-dim because of the "pion bump" falling off below ~100 MeV. We investigate the MeV background from star-forming galaxies by running one-zone models of cosmic ray populations, taking into account the leptonic emission, including Inverse Compton (IC) and bremsstrahlung, as well as nuclear lines, emission from core collapse supernovae, and positron annihilation emission, besides the pionic emission. We use the Milky Way and the GeV-TeV detected starbursts M82 and NGC 253 as templates of normal and starburst galaxies, and compare our models to radio and GeV-TeV gamma-ray data. We find that (1) IC losses off the CMB flatten out the pion bump at high z for normal galaxies, (2) we cannot rule out that starbursts have significant MeV emission if their magnetic field strengths are low, and (3) cascades can contribute to the MeV emission of starbursts if they emit mainly hadronic gamma rays. The star-forming galaxy contribution to the GeV background is uncertain by an order of magnitude, depending on how much of the cosmic star-formation is in starbursts. Our fiducial model predicts that ~1/3 of the unresolved GeV background is from star-forming galaxies, with comparable contributions from normal and starburst galaxies. About ~2% of the claimed 1 MeV background is diffuse emission from star-forming galaxies; we place a firm upper limit of ~10% contribution based on the requirement that star-forming galaxies do not overpower the observed gamma-ray background at any energy. The low star-forming galaxy contribution arises because the observed gamma-ray background spectrum steeply falls with energy, while the star-forming contribution slowly increases with energy in the MeV range. A different source class must emit the observed MeV background, if it is real.
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http://arxiv.org/abs/1206.0772
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