2012/06/04 by Nachiketa Chakraborty, Brian D. Fields · 1 citation
Physics and Astronomy · #Amplitude #Astrophysical Phenomena and Observations #Astrophysics and Cosmic Phenomena #Cosmic ray #Dark Matter and Cosmic Phenomena #Fermi Gamma-ray Space Telescope #Galaxy #Interstellar medium #Luminosity #Photon #Supernova #astro-ph.CO #astro-ph.HE
paper · pdf · doi:10.1088/0004-637x/773/2/104
32 pages, 4 figures. Interested readers may wish to consult Lacki, Horiuchi and Beacom (2012), which focusses on the MeV background from galaxies
arxiv created 2012/06/04 · openalex publication_date 2013/07/31 · arxiv updated 2015/06/05 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
Fermi has resolved several star-forming galaxies, but the vast majority of the star-forming universe is unresolved, and thus contributes to the extragalactic gamma-ray background (EGB). Here, we calculate the contribution of star-forming galaxies to the EGB in the Fermi range from 100 MeV to 100 GeV due to inverse-Compton (IC) scattering of the interstellar photon field by cosmic-ray electrons. We first construct one-zone models for individual star-forming galaxies assuming that supernovae power the acceleration of cosmic rays. We develop templates for both normal and starburst galaxies, accounting for differences in the cosmic-ray electron propagation and in the interstellar radiation fields. For both types of star-forming galaxies, the same IC interactions leading to gamma rays also substantially contribute to the energy loss of the high-energy cosmic-ray electrons. Consequently, a galaxy's IC emission is determined by the relative importance of IC losses in the cosmic-ray electron energy budget ("partial calorimetry"). We calculate the cosmological contribution of star-forming galaxies to the EGB using our templates and the cosmic star formation rate distribution. For all of our models, we find that the IC EGB contribution is almost an order of magnitude less than the peak of the emission due to cosmic-ray ion interactions (mostly pionic p cr p ism → π 0 → γγ); even at the highest Fermi energies, IC is subdominant. The flatter IC spectrum increases the high-energy signal of the pionic+IC sum, bringing it closer to the EGB spectral index observed by Fermi . Partial calorimetry ensures that the overall IC signal is relatively well constrained, with only uncertainties in the amplitude and spectral shape for plausible model choices. We conclude with a brief discussion on how the pionic spectral feature and other methods can be used to measure the star-forming component of the EGB.