Search for high-energy γ-ray emission from galaxies of the Local Group with Fermi/LAT

摘要

Context. With the discovery of high-energy γ-ray emission from the Andromeda galaxy (M?31) by the Fermi/LAT collaboration, normal galaxies begin to arise from the shadows for the first time, providing insight into cosmic ray acceleration in external galaxies. Aims. We search for high-energy γ-ray emission from those galaxies in the Local Group that have so far not been investigated: M?81, M?83, IC?342, Maffei?1, Maffei?2, and M?94. Methods.Fermi/LAT public data from August 4, 2008 to January 1, 2011 were analysed for these galaxies. We compared the results to other starburst and normal galaxies detected so far at high energies: the Magellanic clouds, M?31, and the starburst galaxies M?82 and NGC?253. Results. No significant detection is found in the data for the sources in our sample, and we derive upper limits on their photon flux. After comparing the results to other Local Group objects, we find that the derived upper limits are fully compatible with expectations from cosmic rays interacting with the interstellar medium within the host galaxies. In the case of M?33 and M?83, a detection in Fermi/LAT data should be imminent. The expected fluxes for the other sources in the sample are below the sensitivity of Fermi/LAT, even after 10?years of observation. Collective emission from compact objects in the host galaxies is also found to be negligible compared to the expected emission from cosmic ray interactions.