THE OPTICAL POLARIZATIONS OF HIGH-REDSHIFT AND INTERMEDIATE-REDSHIFT RADIO GALAXIES

Optical polarization measurements are presented for 12 high- and intermediate-redshift radio galaxies (0.2 < z < 0.85). The data confirm that large optical polarization is relatively common at high redshifts: five out of seven of the z > 0.5 objects with good polarization measurements have their integrated light polarized at the 5-20 per cent level. This result, and the fact that the polarization is frequently oriented perpendicular to the radio axis, suggests that a large fraction of the UV continuum has been scattered out of radiation beams from the AGN by dust or electrons in the high-redshift host galaxies. In contrast, we find that none of the intermediate-redshift (0.2 < z < 0.5) objects shows polarization at the > 5 per cent level. It is likely that the implied redshift dependence is largely a consequence of the varying dilution of the polarization by the light from the stars in the haloes of the galaxies - the dilution is least for the high-redshift objects because they are observed in the rest-frame UV - but it is also possible that the scattering ISM is more effective and/or the radiation beams are stronger at high redshifts.