Multi-frequency polarization properties of ten quasars on deca-parsec
  scales at z > 3
  Global VLBI (EVN+VLBA) polarization observations at 5 and 8.4 GHz of ten high
redshift (z > 3) quasars are presented. The core and jet brightness
temperatures are found through modelling the self-calibrated uv-data with
Gaussian components, which provide reliable estimates of the flux density and
size of individual components. The observed high core brightness temperatures
(median $T_{\rm b,\,core}=4\times10^{11}$ K) are consistent with Doppler
boosted emission from a relativistic jet orientated close to the line-of-sight.
This can also explain the dramatic jet bends observed for some of our sources
since small intrinsic bends can be significantly amplified due to projection
effects in a highly beamed relativistic jet. We also model-fit the polarized
emission and, by taking the minimum angle separation between the model-fitted
polarization angles at 5 and 8.4 GHz, we calculate the minimum inferred Faraday
rotation measure (RM$_{\rm min}$) for each component. We also calculate the
minimum intrinsic RM in the rest frame of the AGN (RM$_{\rm min}^{\rm intr}$ =
RM$_{\rm min} (1+z)^2$), first subtracting the integrated (presumed foreground)
RM in those cases where we felt we could do this reliably. The resulting mean
core $|$RM$_{\rm min}^{\rm intr}|$ is 5580 rad m$^{-2}$, with a standard
deviation of 3390 rad m$^{-2}$, for four high-z quasars for which we believe we
could reliably remove the foreground RM. We find relatively steep core and jet
spectral index values, with a median core spectral index of -0.3 and a median
jet spectral index of -1.0. Comparing our results with RM observations of more
nearby Active Galactic Nuclei at similar emitted frequencies does not provide
any significant evidence for dependence of the quasar nuclear environment with
redshift.
