Multifrequency polarization properties of 10 quasars on decaparsec scales at z > 3

Abstract

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.