Ultraviolet spectropolarimetry of narrow-line radio galaxies

We present the results of UV spectropolarimetry (lambda (rest) similar or e qual to 2000-3000 Angstrom) and far-UV spectroscopy (lambda (rest) similar or equal to 1500-2000 Angstrom) of two low-redshift narrow-line radio galax ies (NLRGs) taken with the Faint Object Spectrograph on board the Hubble Sp ace Telescope (HST). Spectropolarimetry of several NLRGs has shown that, by the presence of broad permitted lines in polarized flux spectrum, they hav e hidden quasars seen through scattered light. Imaging polarimetry has show n that NLRGs including our targets often have large scattering regions of a few to greater than or similar to 10 kpc scale. This has posed a problem c oncerning the nature of the scatterers in these radio galaxies. Their polar ized continuum has the spectral index similar to or no bluer than that of q uasars, which favors electrons as the dominant scattering particles. The la rge scattering region size, however, favors dust scattering because of its higher scattering efficiency compared to electrons. In this paper we invest igate the polarized flux spectrum over a wide wavelength range, combining o ur UV data with previous optical/infrared polarimetry data. We infer that t he scattering would be often caused by opaque dust clouds in the NLRGs and this would be part of the reason for the apparently gray scattering. In the high-redshift radio galaxies, these opaque clouds could be the protogalact ic subunits inferred to be seen in the HST images. However, we still cannot rule out the possibility of electron scattering, which could imply the exi stence of a large gas mass surrounding these radio galaxies.