HST Ultraviolet Spectropolarimetry of NGC 1068

Abstract

We present multiaperture spectropolarimetric observations of the Seyfert 2 galaxy NGC 1068. The nuclear region of NGC 1068 was observed with three apertures 4.3" x 1.4", 1.0" and 0.3") by the Hubble Space Telescope. These ultraviolet data allow us to study 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 & Miller (1985) and Code et al. (1993). Shortward of 2700 A, the continuum polarization is constant, P ~ 16%, at a position angle of 97^deg^, 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, Mg II λλ2796,2804 and C III] λ1909, are seen in polarized flux (Lyα and C IV are not included in our spectral region). The total flux data also indicate that from ~2200 A to ~3000 A the spectrum of NGC 1068 is strongly affected by blended Fe II emission. These polarimetry data are explained by the occultation/reflection model proposed by Antonucci & 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 axis. Radiation from the broad-line region and featureless continuum source can escape along the poles of the torus where it is scattered to us. This scattered light receives a partial linear polarization, perpendicular to the symmetry axis of the torus. Our measurements of the relative fluxes and polarized fluxes in the three apertures indicate that the scattering region is spatially extended on a scale of ~1". Also, in the 0.3" aperture, and to a lesser extent in the 1.0" aperture, the polarization is somewhat higher, and the position angle somewhat lower, than in the largest aperture. Presumably, isolating the central part of the reflecting region results in less geometrical cancellation of the polarization and an angle indicative of the location of the innermost scattering particles. As a result of these observations, a UV spectrum was obtained which is far better than any previously obtained for a Seyfert 2.