Astron. Astrophys. 329, 809-820 (1998)
- Multifrequency radio observations of 15 radio galaxies at z
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 6000 rad m-2
detected in 1138-262 (z = 2.17).
- The observations suggest that the extended medium around the
sources is magnetised with 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 25 and
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 2 may be good candidates for
targetted searches for high redshift clusters.
© European Southern Observatory (ESO) 1998
Online publication: December 16, 1997