Ultraviolet Spectropolarimetry of Narrow‐Line Radio Galaxies

Abstract

We present the results of UV spectropolarimetry and far-UV spectroscopy (j rest^2 000È3000 A ) of two low-redshift narrow-line radio galaxies (NLRGs) taken with the Faint (j rest^1 500È2000 A ) Object Spectrograph on board the Hubble Space T elescope (HST ). Spectropolarimetry of several NLRGs has shown that, by the presence of broad permitted lines in polarized Ñux spectrum, they have hidden quasars seen through scattered light. Imaging polarimetry has shown that NLRGs including our targets often have large scattering regions of a few to kpc scale. This has posed a problem concerning the Z10 nature of the scatterers in these radio galaxies. Their polarized continuum has the spectral index similar to or no bluer than that of quasars, which favors electrons as the dominant scattering particles. The large scattering region size, however, favors dust scattering because of its higher scattering efficiency compared to electrons. In this paper we investigate the polarized Ñux spectrum over a wide wavelength range, combining our UV data with previous optical/infrared polarimetry data. We infer that the 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 protogalactic subunits inferred to be seen in the HST images. However, we still cannot rule out the possibility of electron scattering, which could imply the existence of a large gas mass surrounding these radio galaxies.

VLT spectropolarimetry of two powerful radio galaxies atz∼ 1.4: ultraviolet continuum, emission-line properties and the nature of high-redshift dust

“…It is found that, for both galaxies, the polarized spectrum is slightly redder than the incident one, which is in agreement with spectropolarimetric studies of other high‐redshift radio galaxies (e.g. Cimatti et al 1996; Kishimoto et al 2001). The ratio between polarized and incident spectra can be directly compared with the scattering efficiency × intrinsic polarization (cf.…”

Subaru Spectropolarimetry of Markarian 573: The Hidden High-Ionization Nuclear Emission-Line Region inside the Dusty Torus

“…This dichotomy can be understood as follows: because high-velocity photoionized clouds in broad-line regions (BLRs) are surrounded by an optically thick, dusty torus, the BLR can be seen only when we view the AGN face-on toward the dusty torus, and they are hidden by the torus when our view is edge-on. This scheme, termed a unified AGN model (see Antonucci 1993 for a review), is supported by a variety of observational findings, including the detection of hidden BLRs in some type 2 AGNs observed in polarized light (e.g., Antonucci & Miller 1985;Miller & Goodrich 1990;Tran et al 1992Tran et al , 1995Tran et al , 2000Young et al 1993;Tran 1995Tran , 2001Young et al 1996;Kay & Moran 1998;Barth et al 1999aBarth et al , 1999bKishimoto et al 2001;Lumsden et al 2001;Nagao et al 2004). This unified model now provides one of the most important frameworks for AGN studies.…”

Spectropolarimetry of high-redshift obscured and red quasars

“…While scattering off of dust or electrons is the most likely source of polarization in AGN, especially at the high levels we observe in our objects, without some understanding from imaging of the scales on which polarization occurs it is difficult to distinguish between the two. For objects where the scattering cone is visible on ∼kpc scales (Dey et al 1996;Kishimoto et al 2001;Zakamska et al 2005) the dominant scatterer is likely dust. In these cases, the required gas mass for electron scattering is likely prohibitively high though it is not always possible to completely rule out electron scattering.…”