The Faraday screen near the nucleus of the CSS quasar 3C 138

Compact Steep Spectrum (CSS) sources are known to show strong optical line emission [CITE] which is generally considered to be due to the interaction between the radio jet and the ISM. This argument is strengthened by the HST observations of [CITE] which show this emission is frequently associated with the radio jet. Previous observations of the CSS quasar 3C 138 [CITE] have shown that the bulk of the jet has very low amounts of Faraday rotation whereas the inner jet shows very high Faraday rotation ( rad m-2). The inner jet is dominated by moving components which allows observing the fine scale of the Faraday screen as the components move behind it. We report the results of such a monitoring campaign which indicated that there is structure in the Faraday screen on the pc and possibly sub pc scale. Motions of the polarized component slowed dramatically during the course of these observations, limiting the region probed. The observations also strongly suggest that the polarized emission from the inner jet is seen through limited holes in a dense Faraday screen. In this case, the observed variations in Faraday rotation are possibly the result of jet–ISM interactions.