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Astron. Astrophys. 325, 493-501 (1997)

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Dual frequency VLBI polarimetric observations of 3C138

W.D. Cotton 1, D. Dallacasa 2, C. Fanti 2, 3, R. Fanti 2, 3, A.R. Foley 4, R.T. Schilizzi 5, 6 and R.E. Spencer 7

1 National Radio Astronomy Observatory* ,520 Edgemont Road, Charlottesville, VA 22903-2475, USA
2 Istituto di Radioastronomia del CNR, Via P. Gobetti 101, I-40129 Bologna, Italy
3 Dipartimento di Fisica, Universitá di Bologna, via Irnerio 46, I-40126 Bologna, Italy
4 Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
5 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
6 Sterrenwacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
7 NRAL-Jodrell Bank, University of Manchester, Macclesfield Cheshire, SK11 9DL, UK

Received 16 December 1996 / Accepted 24 March 1997


We report VLBI polarimetric observations of the steep spectrum compact radio source 3C138. The relatively small range of Faraday rotation measured in the outer jet and hotspot region suggests that the jet is not being confined by a dense plasma. These results are inconsistent with the "frustrated jet" scenario and are consistent with the "young source" scenario. Evidence is given for extensive plasma in the immediate region of the nucleus. The nuclear region is depolarized at 1.7 GHz and the high value of -1781 [FORMULA] 48 rad/m2 is measured for the rotation measure at a projected distance of 22/h pc from the nucleus. This suggests that the ionized gas giving rise to the broad emission lines may be confined to the region immediately surrounding the nucleus. Comparison with earlier results indicates that two knots in the inner jet are separating at an apparent rate of 6.3c/h. 1

Key words: techniques: polarimetric – galaxies: quasars: individual: 3C138 – galaxies: jets – radio continuum: galaxies

Send offprint requests to: W.D. Cotton

*The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities Inc., under cooperative agreement with the National Science Foundation.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998