High-energy emission from star-forming galaxies
  Adopting the convection-diffusion model for energetic electron and proton
propagation, and accounting for all the relevant hadronic and leptonic
processes, the steady-state energy distributions of these particles in the
starburst galaxies M82 and NGC253 can be determined with a detailed numerical
treatment. The electron distribution is directly normalized by the measured
synchrotron radio emission from the central starburst region; a commonly
expected theoretical relation is then used to normalize the proton spectrum in
this region, and a radial profile is assumed for the magnetic field. The
resulting radiative yields of electrons and protons are calculated: the
predicted >100MeV and >100GeV fluxes are in agreement with the corresponding
quantities measured with the orbiting Fermi telescope and the ground-based
VERITAS and HESS Cherenkov telescopes. The cosmic-ray energy densities in
central regions of starburst galaxies, as inferred from the radio and gamma-ray
measurements of (respectively) non-thermal synchrotron and neutral-pion-decay
emission, are U=O(100) eV/cm3, i.e. at least an order of magnitude larger than
near the Galactic center and in other non-very-actively star-forming galaxies.
These very different energy density levels reflect a similar disparity in the
respective supernova rates in the two environments. A L(gamma) ~ SFR^(1.4)
relationship is then predicted, in agreement with preliminary observational
evidence.
