Radio Haloes Around Galaxies and in Clusters

Abstract

The question of whether or not our and other normal galaxies have some sort of halo — an extended region containing, in particular, cosmic rays — has been discussed for no less than 25 years. Such a “cosmic ray halo” (CRH) appears as a radio-halo, although the absence of the latter is not evidence against the presence of CRH. The point is that the relativistic electrons responsible for the radio emission from the radio-halo undergo synchrotron and Compton losses which are practically absent in the case of the cosmic-ray proton-nuclear component. Possibly because the discussion concerning the existence of the radio-halo in the Galaxy has lasted for years it has acquired a particular character. The latter is clearly reflected in the report by Baldwin (1976) who emphasized that: “In this discussion so far I have avoided the use of the phrase “radio-halo”. It arouses antagonism in otherwise placid astronomers and many sought to deny its existence…” Such a situation evidently reflects the difficulties that arise in detecting the radio-halo of our own Galaxy when account is taken of other confusing galactic sources as well as of the metagalactic background.