HST Ultraviolet Spectropolarimetry of NGC 1068

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Session 4 -- AGNs
Display presentation, Monday, 9:20-6:30, Pauley Room

[4.01] HST Ultraviolet Spectropolarimetry of NGC 1068

T. Hurt, R.R.J. Antonucci (UC Santa Barbara), J.S. Miller (UC Obs.)

Multi-aperture HST spectropolarimetric observations of the Seyfert 2 galaxy NGC 1068 are discussed in the context of the occultation/reflection model for Seyfert galaxies. These UV observations allow us to answer questions about the scattered, nuclear light without the complication of polarization dilution caused by strong unpolarized starlight. In particular, we can determine the mechanism of the scattering and the size scale of the scattering region.

Our data confirm the findings of Antonucci and Miller (1985) and Code et al (1993). Shortward of 2700\AA, the continuum polarization in a 4.3${\tt ''}$ x 1.4${\tt ''}$ aperture is constant, P $\sim$ 16\%, at a postion angle of $\sim$96$^\circ$, indicating electron scattering as the mechanism for the polarization. In addition, the narrow line emission is much less polarized than the continuum, and two broad lines, MgII $\lambda\lambda$2796, 2804 and CIII] $\lambda$1909, have large equivalent widths in polarized flux. (Ly$\alpha$ and CIV are not included in our spectral region.) These findings are explained by the occultation/ reflection model proposed by Antonucci and Miller (1985). According to this model, NGC 1068 harbors a Seyfert 1 nucleus which is obscured along the line of sight by an opaque torus. The symmetry axis of the torus is aligned with the radio jet. Radiation from the continuum source and BLR can escape along the poles of the torus where it is scattered to us, giving this radiation a partial linear polarization.

Our measurements of the relative fluxes in the three apertures indicate that the extent of the reflecting region is $\sim$1.0${\tt ''}$, corresponding to a size of $\sim$100 pc. In the 0.3${\tt ''}$ aperture, and to a lesser extent in the 1.0${\tt ''}$ aperture, P is somewhat higher, and $\theta$ somewhat lower than in the largest aperture. Presumably, isolating the central part of the reflecting region results in less geometrical dilution of P and a more ``instantaneous'' value of the nuclear axis.

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