High Energy Gamma-ray Absorption and Cascade Emission in Nearby
  Starburst Galaxies
  High energy gamma-ray emission from two nearby bright starburst galaxies, M82
and NGC 253, have recently been detected by Fermi, H.E.S.S., and VERITAS. Since
starburst galaxies have a high star formation rate and plenty of dust in the
central starburst region, infrared emissions are strong there. Gamma-ray
photons are absorbed by the interstellar radiation field photons via electron
and positron pair creation. The generated electron and positron pairs up
scatter the interstellar photons to very high energy gamma-ray photons via
cascade emission through inverse Compton scattering. In this paper, we evaluate
the contribution of this cascade emission to the gamma-ray spectra of M82 and
NGC 253. Although it would be difficult to see direct gamma- ray evidence of
cosmic-rays with an energy > 10 TeV due to the gamma-ray attenuation, the
resulting cascade emission would be indirect evidence. By including the cascade
component, we find that the total flux above 1 TeV increases ~18% and ~45%
compared with the absorbed flux assuming the maximum kinetic proton energy as
45.3 TeV and 512 TeV, respectively. Future gamma-ray observatories such as CTA
would be able to see the indirect evidence of cosmic-ray with an energy > 10
TeV by comparing with theoretical emission models including this cascade
effect.
