STELLAR AND SCATTERED-LIGHT IN A RADIO GALAXY AT Z=2.631

We present the results of optical imaging polarimetry of a radio galaxy at z = 2.63. High linear polarization (P = 8.3 +/- 2.3%) is measured in the rest frame UV continuum between 1600 and 2100 angstrom. The orientation of the electic vector is perpendicular to the UV and Lyalpha axis and, as for the other known polarized distant radio galaxies, scattering of anisotropic nuclear radiation is the favored polarization mechanism. We discuss modeling of the observed degree of polarization and of the spectral energy distribution of the galaxy as given by optical and infrared photometry, with evolved stars, young stars, anisotropic nuclear light scattered by dust or electrons, and direct nuclear radiation. The presence of the scattered component is required by the high UV polarization. Nevertheless it is not possible to exclude young stars or direct nuclear radiation solely on the basis of the photometric fitting. Also an evolved stellar population is necessary to explain the infrared colors. We discuss a possible test of the Einstein equivalence principle and of the presence of a light pseudoscalar boson based on the preservation of photon polarization orientation over large distances.