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

Recently, two nearby prominent starburst galaxies, M82 and NGC253, have been detected as point-like sources with gamma-ray telescopes at TeV energies [1] [2]. It has been claimed that these detections show that the cosmic ray intensity in the starburst galaxies is three orders of magnitude higher than in the Milky Way galaxy, assuming that the observed gamma rays arise due to pion production of cosmic rays interacting with the ambient gas. The observed spectrum is flatter than the cosmic ray spectrum in the Milky Way galaxy, and this 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 of cosmic rays according to which the cosmic rays are accelerated in the shells of supernova remnants. As an immediate corollary it follows that the cosmic ray driven gamma ray luminosity should scale with the gas density and supernova rate. At lower energies, gamma-ray measurements with the Fermi LAT instrument could provide support for this scaling [4]. However, there are nagging doubts about the interpretation of the observations at very high energies. At the distance of the observed galaxies, point-like sources cannot be discriminated from 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 the gamma-ray luminosity arises.