The optical polarizations of high- and intermediate-redshift radio galaxies

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Abstract
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 red shift 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.

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