Keck spectropolarimetry of two high-z radio galaxies: Discerning the components of the alignment effect

We present optical spectropolarimetric observations obtained with the W. M. Keck Telescope of two powerful, high-redshift radio galaxies which exhibit radio-optical alignments, 3C 13 (z = 1.351) and 3C 356 (z = 1.079). 3C 13 is fairly strongly polarized in the blue, with the electric vector oriented perpendicular to the major axis of UV continuum emission. 3C 356 is known to have two radio/optical components (labeled a and b) along the radio source axis, but it is unclear which of them is the nucleus of the radio source. Our observations show that both components a and b are polarized with the electric vectors in both cases oriented approximately orthogonal to the optical a-b axis. Component a also shows evidence for broad Mg II lambda 2800 emission both in polarized and total light, while the narrow forbidden lines are unpolarized Our observations support the unified model of powerful radio-loud active galactic nuclei (AGNs) and allow us for the first time to quantify the contribution of the different radiative components to the alignment effect of a high-z radio galaxy (3C 356a): the nonstellar radiation (scattered and nebular continua) constitutes about 80% of the total UV continuum emission at 2800 Angstrom and an evolved stellar population with an age similar to 1.5-2.0 Gyr can account for the remainder of the UV light. We also detect the stellar Ca II K absorption line in the spectra of both components. Although the present data do not clarify unambiguously whether a or b is the nucleus of 3C 356, they suggest that the scenario in which a contains the hidden quasar is energetically more favorable. If the nucleus is located in a, our observations show that electron scattering is plausible, and support the scenario in which 3C 356 is surrounded by an ionized intracluster medium, as suggested by ROSAT observations.