Optical polarization in distant radio galaxies

We review the optical polarization properties of radio galaxies with redshifts greater than 0.1. We find a strong tendency for the E-vector to be either perpendicular or parallel to the radio axis, and perpendicular to the axis in all the high-redshift galaxies. The observed degree of linear polarization is correlated with redshift and radio power, and is anticorrelated with the rest-frame wavelength of the measurement and the radio spectral index. No correlation is found with absolute magnitude. For the high-polarization objects (P ≥ 3 per cent), the polarization also increases with the radio asymmetry parameter Q, which is thought to be related to the interstellar-medium density ratio. After correcting the polarization for stellar dilution, only weak correlations with redshift and rest-frame wavelength remain, suggesting that the redshift dependence could be entirely explained as a wavelength-dependent contrast effect. It is possible to divide the radio galaxies into two groups: the high-redshift objects (z ≥ 0.6) with high perpendicular polarization and strong radio-optical alignments, and the low-redshift sources with low polarization having either parallel or perpendicular E-vectors. The dominant polarization mechanism for the first group must be the scattering of nuclear radiation. We present a dust-scattering model that explains 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° half-opening angle. The required amount of spherically distributed dust is $$(1-3) \times {10}^{8} {\text M}_{\odot}$$ (consistent with the IRAS data), which produces only a small dust extinction (A_v ~ 0.1) along a line of sight to a quasar. The low-redshift radio galaxies do not have a single dominant polarization mechanism. In addition to scattering, polarization by transmission through aligned dust grains and synchrotron radiation can play a role in some of these objects.