An optical and near-infrared polarization survey of Seyfert and broad-line radio galaxies. 2 : the wavelength dependence of polarization

Abstract

We discuss the wavelength dependence (0.44-2.2 micron) of polarization of the sample of 71 Seyfert and three broad-line radio galaxies presented in Paper I. For four galaxies, 3A 0557-353, Fairall 51, IC 4329A and NGC 3753, we also present spectropolarimetry covering the wavelength range of 0.4-0.6 microns. Most objects in our sample show decreasing polarization with increasing wavelength, as might be expected for polarization produced by dichroic absorption or by scattering (when host galaxy dilution is taken into account). Thirteen objects have wavelength-dependent polarization which resembled that produced by the transmission of light through aligned dust grains. Two objects (NGC 1065 and Mkn 704) show a clear increase in polarized flux density towards the blue, as expected for scattering off small dust grains. However, in many objects a similar increase may not be seen, due to reddening. Only eight objects show any significant wavelength dependence of polarization position angle. Seven objects (3C 234, MCG-5-23-16, Tol 0109-353, I11058-1131, NGC 5506, NGC 6300 and I19550-1818) show increasing polarized flux density with increasing wavelength, which could be produced by the scattering of radiation off electrons, or the presence of a non-thermal component. Only four of these objects, all type 2 Seyferts (NGC 1685, MCG-5-23-16, NGC 5506 and I19580-1818), show an increase in degree of polarization to the red. The spectropolarimetric data show that, for the two Seyfert 1 galaxies 3A 0557-383 and Fairall 51, the forbidden lines are less polarized than the continuum and permitted lines.