Polarimetry and Unification of Low-Redshift Radio Galaxies

Abstract

We have made high-quality measurements of the polarization spectra of 13FR II radio galaxies and taken polarization images for 11 of these withthe Keck telescopes. Seven of the eight narrow-line radio galaxies(NLRGs) are polarized, and six of the seven show prominent broad Balmerlines in polarized light. The broad lines are also weakly visible intotal flux. Some of the NLRGs show bipolar regions with roughlycircumferential polarization vectors, revealing a large reflectionnebula illuminated by a central source. Our observations powerfullysupport the hidden quasar hypothesis for some NLRGs. According to thishypothesis, the continuum and broad lines are blocked by a dustymolecular torus, but can be seen by reflected, hence polarized, light.Classification as a NLRG, a broad-line radio galaxy (BLRG), or a quasartherefore depends on orientation. However, not all objects fit into thisunification scheme. Our sample is biased toward objects known in advanceto be polarized, but the combination of our results with the 1996findings of Hill, Goodrich, & DePoy show that at least six out of acomplete, volume and flux-limited sample of nine FR II NLRGs have broadlines, seen either in polarization or Palpha. The BLRGs in our samplerange from 3C 382, which has a quasar-like spectrum, to the highlyreddened IRAS source FSC 2217+259. This reddening sequence suggests acontinuous transition from unobscured quasar to reddened BLRG to NLRG.Apparently the obscuring torus does not have a distinct edge. The BLRGshave polarization images that are consistent with a point sourcebroadened by seeing and diluted by starlight. We do not detect extendednebular or scattered emission, perhaps because it is swamped by thenuclear source. Our starlight-corrected BLRG spectra can be explainedwith a two-component model: a quasar viewed through dust and quasarlight scattered by dust. The direct flux is more reddened than thescattered flux, causing the polarization to rise steeply to the blue.Strong rotations of the electric vector position angle across Halphain 3C 227 and 3C 445 may be explained by systematic orbital motions inan equatorial broad-line region.