LARGE ROTATION MEASURES IN RADIO GALAXIES AT Z-GREATER-THAN-2

We have carried out multifrequency radio polarisation imaging of a sam ple of 15 radio galaxies at z > 2 from the MRC/1Jy sample using the VL A. We report here the discovery of large rotation measures (RM) in a c onsiderable fraction of the high redshift radio galaxies. Using the di fference between the RM values of the two radio lobes in each source a nd statistical arguments, we show that the Faraday screens responsible for the RMs are most likely to be in the vicinity of the radio source s themselves. Four of the 15 galaxies show intrinsic (redshift correct ed) RMs in excess of 1000 rad m(-2) with the highest value of similar to 6000 rad m(-2) in 1138-262 at z = 2.17. These observations suggest that the environments of the radio galaxies at z > 2 have micro gauss magnetic fields correlated over many kpc (>5-10), at least. We have di scussed the problems due to the short time available at those redshift s for the various mechanisms, which are believed to generate and corre late strong magnetic fields on large scales, to operate. In particular , we argue that, unlike at low redshifts, cluster cooling flows are un likely to have a role in forming deep Faraday screens at high redshift s. It is not clear if the dynamo mechanism is capable of generating su ch fields in the ambient medium around the radio sources. It appears p lausible that condensates of magnetised plasma (galactic or subgalacti c sized) are the deep Faraday screens responsible for the observed RMs . We suggest that plasma clumps of as small as 10(9) M. in the path of the radio jet may generate very large RMs. The presence of such stron g and large scale magnetic fields in the medium around the radio sourc es at such early epochs poses a considerable challenge to models of th e origin of magnetic fields in the Universe.