Dual frequency VLBI polarimetric observations of 3C 138.

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 + − 48 rad/m 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