Aleksandra Dobardzic, Tijana Prodanovic
For more than a decade now the complete origin of the diffuse gamma-ray emission background (EGRB) has been unknown. Major components like unresolved star-forming galaxies (making <50% of the EGRB) and blazars (<23%) have failed to explain the entire background observed by Fermi. Another, though subdominant, contribution is expected to come from the process of large-scale structure formation. The growth of structures is accompanied by accretion and merger shocks, which would, with at least some magnetic field present, give rise to a population of structure-formation cosmic rays (SFCR). Any cosmic-ray population results also in gamma-ray emission at some level due to interaction of cosmic-ray protons with ambient hydrogen, where gamma rays come from the decay of neutral pions created in this interaction. The most promising insight into SFCRs was expected to come from Fermi -LAT observations of clusters of galaxies, however only upper limits and no detection have been placed. Here we build a model of gamma-ray emission from large-scale accretion shocks implementing a source evolution calibrated with the Fermi -LAT cluster observation limits. Together with contribution of normal star-forming galaxies, our modeled SFCR gamma-ray emission, is a good fit to the observed EGRB, and can account for the unexplained gamma-ray excess at E >10GeV. Moreover, we show that, even though the gamma-ray emission arising from structure formation shocks at galaxy clusters is below previous estimates, these large scale shocks can still give an important, and even dominant at high energies, contribution to the EGRB. Future detections of cluster gamma-ray emission would make our upper limit of the extragalactic gamma-ray emission from structure-formation process, a firm prediction, and give us deeper insight in evolution of these large scale shock.
View original:
http://arxiv.org/abs/1210.4551
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