Scattered Radiation from Obscured Quasars in Distant Radio Galaxies

We present optical spectropolarimetric and imaging polarimetric observations of four high-redshift radio galaxies (HzRGs) obtained with the Low Resolution Imaging Spectrometer of the 10 m Keck I telescope. A broad Mg II l2800 emission line is detected in the total and polarized flux spectra of 3C 265 and 3C 277.2. The fractional polarization is high, and both it and the position angle are constant with wavelength after accounting for dilution by unpolarized starlight of the host galaxy, which can contribute substantially. An extended unpolarized continuum similar to that observed in other active galactic nuclei is also detected. Imaging polarimetry reveals a rough double-fan morphology of the polarized light coincident with the extended aligned emission regions, with the position angle essentially perpendicular to the radial structure of the extended UV/optical emission and with the degree of polarization increasing with radius away from the nucleus. The radio jets lie inside the extended emission regions and, like every radius, are roughly perpendicular to the polarization position angle. These results strengthen the view that powerful radio galaxies would be called quasars if viewed from the proper direction. Based on the polarimetric data presented in this paper and in previous studies, scattering of radiation from an obscured quasar source appears to be the preferred interpretation over jet-induced star formation for explaining the alignment effect in HzRGs. Both electrons and dust can play a major role in the scattering process. However, the lack of strong direct evidence for either case and our ignorance of the properties and distribution of the scatterers in these galaxies make it very difficult to discriminate between the two. Our data reveal a chance alignment of 3C 343.1 with a foreground galaxy, which dominates the observed optical flux from the system.