Keck spatially resolved spectropolarimetry of the distant radio galaxy 3C 324

We present the results of spatially resolved optical spectropolarimetry of the distant (z = 1.2) radio galaxy 3C 324 obtained with the W. M. Keck telescope. The spatially integrated linear polarization is P approximate to 11% with the electric vector oriented perpendicular to the UV continuum axis. The polarization of the continuum and the position angle of the electric vector are constant with wavelength. In addition, we find that the polarization is spatially extended, but it does not show strong variations along the major axis of the UV continuum. We detect the broad permitted emission line of Mg II lambda 2800 in the polarized spectrum. The polarization of the narrow forbidden emission line of [O II] lambda 3727 is small. Scattering of anisotropic radiation emitted by a hidden quasar nucleus is the favored mechanism to explain the observed properties. The significant reddening required to model the observed total flux and polarized spectra, and to explain the small residual [O II] lambda 3727 line polarization, suggests that a substantial amount of dust is present. Scattering by hot (T-e > 10(6) K) electrons can be ruled out by the velocity width of the polarized Mg II lambda 2800 line. The required masses of scattering particles and the presence of dust in 3C 324 suggest that dust scattering is dominant in individual clouds, but a contribution by warm electron scattering cannot be excluded. Despite the large uncertainties, our data suggest that a fraction of the total UV flux is due to unpolarized radiation emitted in situ and whose nature is presently unclear. We detect a faint companion galaxy along the radio axis of 3C 324 and find that its spectrum shows evidence for ongoing star formation. We discuss the implications of our findings and their relevance to the interpretation of the alignment effect, the unified model of radio-loud active galactic nuclei, and the stellar content of high-redshift radio galaxies.