The B3-VLA CSS sample . IV. kp-scale polarization properties

We discuss VLA polarization measurements at 8.5, 4.9 and 1.4 GHz for the B3-VLA sample of Compact Steep-spectrum Sources (CSS). The measurements at the two higher frequencies were presented in a previous paper (Fanti et al. \cite{fan3}), while those at 1.4 GHz are from the NVSS. The study of the fractional polarization as a function of wavelength shows a variety of behaviours not always accounted for by the current simple models. In a large fraction of sources the integrated rotation angle appears to follow the λ2 law down to 1.4 GHz, although this conclusion has to be treated with some care due to the small number of polarization measurements. For sources resolved at the two higher frequencies we find that a number exhibit asymmetries in the 8.5 GHz fractional polarization, in depolarization and rotation angle. We discuss depolarization effects and rotation of the polarization angle. We find that Faraday depth effects are very strong within 2-3 kpc of the nucleus, as found earlier by Cotton et al. (\cite{cott}). A simple model for an external Faraday screen is able to account for the gross observed features. We also find that the Rotation Measure and the Faraday Dispersion appear to increase with the source red-shift. Tables A1, A2 and A3 are only available in electronic form at http://www.edpsciences.org