Spectropolarimetry of broad H alpha lines in radio galaxies: constraints on the geometry of the broad-line and scattering regions

We present optical spectropolarimetric observations of three radio-loud bro ad-line active galactic nuclei: 3C 332, 3C 351 and 4C 73.18, together with new observations of 3C 390.3, which we observed previously. Intrinsically p olarized broad H alpha emission was detected in 3C332 and 3C390.3 at levels of similar to3 per cent and similar to1 per cent, respectively. Significan t polarization was not detected in 3C 351, while the polarization measured in 4C 73.18 (similar to1 per cent) is consistent with foreground contaminat ion by the galactic interstellar medium. Although 3C 332 and 3C 390.3 both exhibit 'double-peaked' broad H alpha line profiles, their polarization pro perties differ significantly. The polarized H alpha emission in 3C332 has a similar profile to that seen in total flux and the continuum and line emis sion is polarized at 70 degrees to the axis of the radio source. In 3C 390. 3, however, the double peaks seen in total flux are suppressed in polarized flux and the H alpha line and continuum are polarized at similar to5 degre es to the axis of the radio emission. Comparison of these new observations of 3C 390.3 with data obtained 2 years previously reveals significant varia tions in both total and polarized flux. The implications of these observations for the relationship between the geo metry of the broad-line region and that of the scattering medium are discus sed. We argue that models in which the H alpha emission arises in a relativ istically rotating disc cannot easily explain the observed polarization pro perties of 3C 332 and, in particular, 3C 390.3. We consider instead a model in which the line emission originates in a bipolar outflow and present cal culations showing that the observed polarization properties of 3C 390.3 can be broadly explained by scattering from the inner wall of an obscuring tor us, while those of 3C 332 are consistent with scattering by particles situa ted above the poles of the torus. We outline a scheme in which either toroi dal or polar scattering dominates in a given object, depending on the orien tation of its radio jet axis to our line of sight.