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Astron. Astrophys. 329, 809-820 (1998)

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6. Conclusions

  • Multifrequency radio observations of 15 radio galaxies at z [FORMULA] 2 from the MRC/1Jy sample have resulted in the detection of large RMs. The Faraday screens responsible for the RM are most likely to be situated in the vicinity of the radio sources. Several sources have values in excess of 1000 rad m-2 with the highest RM of [FORMULA] 6000 rad m-2 detected in 1138-262 (z = 2.17).
  • The observations suggest that the extended medium around the sources is magnetised with [FORMULA] G fields correlated on scales of many kpc.
  • Unlike at low redshifts, cluster cooling flows are unlikely to have a significant role in forming deep Faraday screens, since only galaxy-sized mini cooling flows (between [FORMULA] 25 and [FORMULA] 45 kpc) are possible at those redshifts, given the constraints of timescale and smooth infall.
  • These observations pose a stiff challenge to the various mechanisms which have been proposed for generating magnetic fields, primarily due to the small time available for amplification of the initial seed field.
  • We suggest that the the Faraday screens of these HRRGs are other collapsed galactic or subgalactic sized objects in the environs of the radio sources. Larger RMs may be produced if such objects are in the path of the radio jet; the collision and the subsequent shearing of such objects may result in the formation of deep Faraday screens capable of generating RMs of thousands of rad m-2.
  • If this scenario is indeed correct, the vicinity of the galaxies with large RMs at z [FORMULA] 2 may be good candidates for targetted searches for high redshift clusters.
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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997