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Astron. Astrophys. 344, 402-408 (1999)

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5. Conclusions

We have presented the results of our analysis of quasi-simultaneous (within [FORMULA] minutes) polarization-sensitive VLBA observations of 3C 119 at three separate frequencies within the available 8-GHz band at the VLBA antennas. These observations have allowed us to derive the distribution of the rotation-measure of 3C 119 on parsec scales.

The main features in the VLBI total intensity structure of 3C 119 at 8-GHz are the core A, a weak jet component B in structural position angle [FORMULA] from A, the brightest component C further along the jet in this same position angle, and the much more extended component D to the south of these features. The rotation measure in 3C 119 is concentrated around component C, which was tentatively identified by Nan et al. (1991) as the site of a collision between the VLBI jet and a dense external medium. The resolution of our measurements enabled the detection of a large rotation-measure gradient of approximately 2300 rad/m2/mas across component C. The direction of this gradient coincides with the inferred direction of the flow of jet material into component C. The concentration of the high-rotation-measure region around C and, especially, the observed gradient of the rotation measure in C, which has a clear relation to the source structure, convincingly demonstrates that the thermal plasma giving rise to the large rotation measure of 3C 119 is located near the source, rather than in our Galaxy. The fact that the rotation measure increases toward the leading edge of C provides support for the deflection hypothesis of Nan et al. (1991), in which the VLBI jet is deflected at C toward the southern component D.

The rotation-measure distribution derived can be used to "derotate" the observed [FORMULA] vectors for the VLBI polarization distribution, enabling a determination of the intrinsic magnetic-field directions on parsec scales. The polarized flux on milliarcsecond scales is dominated by component C. The inferred magnetic field B is aligned with the directions of local flows in the region of component C. B bends smoothly through roughly [FORMULA] from the northeastern to the southwestern edge of component C, from the direction backward toward component B toward the direction forward toward component D. In addition, the rotation-measure gradient is already clearly seen even in the northeast of C, where B has not yet begun to rotate, indicating that the rotation-measure gradient is primarily associated with a change in the electron density, rather than with a change in the component of B along the line of sight. This behavior is also consistent with the deflection hypothesis of Nan et al. (1991).

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999