Spectropolarimetry of Seyfert 2 Galaxies with Obscured Broad-Line Regions

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Session 100 -- AGN: Unified Picture
Oral presentation, Friday, January 14, 2:15-3:45, Crystal Forum Room (Crystal City Marriott)

[100.04] Spectropolarimetry of Seyfert 2 Galaxies with Obscured Broad-Line Regions

H. D. Tran (UCO/Lick Observatory)

We present high signal-to-noise ratio spectropolarimetric observations of ten Seyfert 2 galaxies currently known to harbor obscured broad-line regions (BLRs). The data, covering the spectral range $\lambda\lambda$ 3200--7400 at moderate resolution, were obtained with the Kast double spectrograph in combination with the CCD polarimeter on the Shane 3-m telescope at Lick Observatory. They indicate that electron scattering is the dominant mechanism responsible for the nuclear polarizations in 9 of the 10 galaxies in the sample, although a substantial contribution from dust scattering and/or transmission is likely in at least 4 objects. In only one case, NGC 7674, does the evidence suggest that dust scattering is the dominant cause of the observed nuclear polarization.

The corrected continuum polarizations display, in all galaxies but NGC 1068, a rise in the amount of polarization in the broad wings of H$\beta$ and H$\alpha$, while the polarization position angle remains more or less the same between the broad-line wings and continuum. In a majority of the objects, the polarization electric vectors are essentially perpendicular to the radio structure axis and/or the position angle of the extended ionization cones or jets. The polarized broad lines tend to be redshifted relative to the galactic nucleus. These observational results strongly suggest that the obscuring torus picture with the general scattering geometry proposed for NGC 1068 is basically correct, but in addition to the scattered (hence polarized) continuum radiation, there must exist a substantial unpolarized component $F_{c2}$ underlying the total observed continua of these Seyfert 2 galaxies. The continuum polarizations after the removal of this diluting component are generally very high ($\ge$ 15\%), and in accord with those expected from the obscuring torus/scattering electron model. Available evidence also suggests that $F_{c2}$ may represent the optically thin thermal radiation from the scattering region itself. Barring strong selection effects, polarized BLRs appear to be found predominantly in multiple, perhaps interacting systems of galaxies, indicating that perturbing or merging processes may play an important role in the existence and/or detection of hidden BLRs in Seyfert 2 galaxies at some stages of their evolution.

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