On the origin of the ultraviolet continuum emission from the high-redshift radio galaxy 3C 256

We report spectropolarimetric observations obtained with the W. M. Keck Telescope of the high-redshift (z = 1.824) aligned radio galaxy 3C 256. Our observations confirm that the spatially extended UV continuum emission from this galaxy is polarized (P-v approximate to 10.9% +/- 0.9%) with the electric vector perpendicular to the aligned radio and optical major axes (theta(v) approximate to 48.0 degrees +/- 2.4 degrees). This strongly suggests that a significant fraction of the rest frame UV continuum emission from the galaxy is not starlight, but is instead scattered light from a powerful active galactic nucleus that is hidden from our direct view. The narrow emission lines, including Mg II, are unpolarized. The percentage polarization of the continuum emission and the polarization position angle are roughly constant as a function of wavelength. Although the present data do not permit us to discriminate between cool electrons and dust as the origin of the scattering, scattering by a population of hot (T greater than or similar to 10(7) K) electrons cannot be the dominant process, since such a population would overproduce X-ray emission. A large population of cooler electrons (T approximate to 10(4) K) could be responsible for both the line emission and the scattered light, but would require that the dust-to-gas ratio in the scattering cones is 10(-3) times smaller than that in our Galaxy and would imply that a large fraction of the baryonic mass in the galaxy is in the ionized component of its interstellar medium. Dust scattering is more efficient, but would result in detectable extinction of the emission-line spectrum unless the dust distribution is more highly clumped than the line emitting gas. Finally, we detect a strong (W-lambda(rest) approximate to 12 Angstrom) broad (FWHM approximate to 6500 km s(-1)) absorption line centered at lambda(rest) approximate to 1477 Angstrom. We discuss several possibilities for its origin and conclude that the most likely candidate is absorption by a high-velocity broad absorption line cloud near the nucleus of 3C 256.