Abstract
The Fermi Gamma-ray Space Telescope has revealed a diffuse γ-ray background at energies from 0.1 gigaelectronvolt to 1 teraelectronvolt, which can be separated into emission from our Galaxy and an isotropic, extragalactic component1. Previous efforts to understand the latter have been hampered by the lack of physical models capable of predicting the γ-ray emission produced by the many candidate sources, primarily active galactic nuclei2-5 and star-forming galaxies6-10, leaving their contributions poorly constrained. Here we present a calculation of the contribution of star-forming galaxies to the γ-ray background that does not rely on empirical scalings and is instead based on a physical model for the γ-ray emission produced when cosmic rays accelerated in supernova remnants interact with the interstellar medium11. After validating the model against local observations, we apply it to the observed cosmological star-forming galaxy population and recover an excellent match to both the total intensity and the spectral slope of the γ-ray background, demonstrating that star-forming galaxies alone can explain the full diffuse, isotropic γ-ray background.
Highlights
A SFG’s diffuse γ-ray emission depends primarily on three factors: its total star formation rate, the distribution of γ-ray energies produced when individual CRs collide with ISM nuclei, and the fraction of CRs that undergo inelastic collisions before escaping the galaxy
In the Methods we describe a new technique to use this model to compute fcal(E), and the total γ-ray emission produced by CR ions in SFGs
The model includes the attenuation of γ-rays produced by both CR ions and leptons due to pair production in collisions with far-infrared photons inside the source galaxy and extragalactic background light photons outside the galaxy, which become important at energies 100 GeV
Summary
We describe our methods to compute γ-ray emission from a single SFG due to both CR ions and leptons, to determine the flux received at Earth from that galaxy, and to apply these models to the CANDELS sample, as well as the details of the Monte Carlo estimation for low redshift source counts. In these expressions, σT is the Thomson cross section, α is the fine structure constant, me is the electron mass, γ = Ec/mec[2] is the electron Lorentz factor, Epeak is the energy where the infrared background peaks (derived from the dust temperature Tdust = 98 (1 + z)−0.065 + 6.9 log M ∗/M∗ and injected as a diluted modified black body spectrum which peaks in photon number at Ep√eak = 2.82 kBTdust where kB is the Boltzmann constant), B = VAi/ 4πnHμpmHχ is the magnetic field strength, urad is the radiation energy density (which based on empirwhere dNγ /dEγ is the total γ-ray production from both CR ions and electrons evaluated at an energy Eγ(1 + z), and dL (z) is the luminosity distance of the source. Since this comparison requires the far-infrared luminosity, we convert the star formation rate to an farinfrared luminosity in the 8−1000 μm band using the relation in Refs. 7,55, corrected to a Chabrier IMF 56; this conversion is valid for star formation rates 1 M yr−1, which encompasses almost all of the observed sample to which we wish to compare
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