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

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Large rotation measures in radio galaxies at z [FORMULA] 2

R.M. Athreya 1, V.K. Kapahi 1, P.J. McCarthy 2 and W. van Breugel 3

1 National Centre for Radio Astrophysics (TIFR) P.O.Bag 3, Pune University Campus, Pune 411 007, India
(ramana@gmrt.ernet.in (RMA) and vijay@gmrt.ernet.in (VKK))
2 Observatories of the Carnegie Institution of Washington, 818, Santa Barbara St., Pasadena, CA 91107, USA
3 IGPP, Lawrence Livermore National Laboratories, P.O.Box 808, Livermore, CA 94550, USA

Received 20 November 1996 / Accepted 26 June 1997


We have carried out multifrequency radio polarisation imaging of a sample of 15 radio galaxies at z [FORMULA] 2 from the MRC/1Jy sample using the VLA. We report here the discovery of large rotation measures (RM) in a considerable fraction of the high redshift radio galaxies. Using the difference between the RM values of the two radio lobes in each source and statistical arguments, we show that the Faraday screens responsible for the RMs are most likely to be in the vicinity of the radio sources themselves. Four of the 15 galaxies show intrinsic (redshift corrected) RMs in excess of 1000 rad m-2 with the highest value of [FORMULA] 6000 rad m-2 in 1138-262 at z = 2.17. These observations suggest that the environments of the radio galaxies at z [FORMULA] 2 have micro gauss magnetic fields correlated over many kpc ([FORMULA] 5-10), at least.

We have discussed the problems due to the short time available at those redshifts for the various mechanisms, which are believed to generate and correlate strong magnetic fields on large scales, to operate. In particular, we argue that, unlike at low redshifts, cluster cooling flows are unlikely to have a role in forming deep Faraday screens at high redshifts. It is not clear if the dynamo mechanism is capable of generating such fields in the ambient medium around the radio sources. It appears plausible that condensates of magnetised plasma (galactic or subgalactic sized) are the deep Faraday screens responsible for the observed RMs. We suggest that plasma clumps of as small as [FORMULA] in the path of the radio jet may generate very large RMs.

The presence of such strong and large scale magnetic fields in the medium around the radio sources at such early epochs poses a considerable challenge to models of the origin of magnetic fields in the Universe.

Key words: galaxies: magnetic fields – radio continuum: galaxies – cosmology: observations

Send offprint requests to: R.M. Athreya

© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997