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Recently, two nearby prominent starburst galaxies, M82 and NGC253, have beendetected as point-like sources with gamma-ray telescopes at TeV energies [1][2]. It has been claimed that these detections show that the cosmic rayintensity in the starburst galaxies is three orders of magnitude higher than inthe Milky Way galaxy, assuming that the observed gamma rays arise due to pionproduction of cosmic rays interacting with the ambient gas. The observedspectrum is flatter than the cosmic ray spectrum in the Milky Way galaxy, andthis could be due to the much higher gas density in the starburst galaxies [3].The interpretation seems to be in line with the Ginzburg-model of the origin ofcosmic rays according to which the cosmic rays are accelerated in the shells ofsupernova remnants. As an immediate corollary it follows that the cosmic raydriven gamma ray luminosity should scale with the gas density and supernovarate. At lower energies, gamma-ray measurements with the Fermi LAT instrumentcould provide support for this scaling [4]. However, there are nagging doubtsabout the interpretation of the observations at very high energies. At thedistance of the observed galaxies, point-like sources cannot be discriminatedfrom diffuse emission for an angular resolution of the order of 0.1 deg. Hence,the question about the contribution of unresolved point-like sources to thegamma-ray luminosity arises.

Gamma-rays from Pulsar Wind Nebulae in Starburst galaxies (HEAD 2010)