OPTICAL POLARIZATION IN DISTANT RADIO GALAXIES

We review the optical polarization properties of radio galaxies with r edshifts greater than 0.1. We find a strong tendency for the E-vector to be either perpendicular or parallel to the radio axis, and perpendi cular to the axis in all the high-redshift galaxies. The observed degr ee of linear polarization is correlated with redshift and radio power, and is anticorrelated with the rest-frame wavelength of the measureme nt and the radio spectral index. No correlation is found with absolute magnitude. For the high-polarization objects (P greater-than-or-equal -to 3 per cent), the polarization also increases with the radio asymme try parameter Q, which is thought to be related to the interstellar-me dium density ratio. After correcting the polarization for stellar dilu tion, only weak correlations with redshift and rest-frame wavelength r emain, suggesting that the redshift dependence could be entirely expla ined as a wavelength-dependent contrast effect. It is possible to divi de the radio galaxies into two groups: the high-redshift objects (z gr eater-than-or-equal-to 0.6) with high perpendicular polarization and s trong radio-optical alignments, and the low-redshift sources with low polarization having either parallel or perpendicular E-vectors. The do minant polarization mechanism for the first group must be the scatteri ng of nuclear radiation. We present a dust-scattering model that expla ins the structure, the polarization properties and the spectral energy distribution of the ultraviolet aligned light with optically thin Mie scattering of quasar radiation emitted in a cone of 47-degrees half-o pening angle. The required amount of spherically distributed dust is ( 1-3) x 10(8) M. (consistent with the IRAS data), which produces only a small dust extinction (A(V) approximately 0.1) along a line of sight to a quasar. The low-redshift radio galaxies do not have a single domi nant polarization mechanism. In addition to scattering, polarization b y transmission through aligned dust grains and synchrotron radiation c an play a role in some of these objects.