HST ultraviolet spectropolarimetry of NGC 1068

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Title:		HST ultraviolet spectropolarimetry of NGC 1068
Authors:		Antonucci, Robert; Hurt, Todd; Miller, Joseph
Affiliation:		AA(Univ. of California, Santa Barbara, CA, US), AB(Univ. of California, Santa Barbara, CA, US), AC(Univ. of California, Santa Barbara, CA, US)
Publication:		The Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 430, no. 1, p. 210-217 (ApJ Homepage)
Publication Date:		07/1994
Category:		Astronomy
Origin:		STI
NASA/STI Keywords:		ASTRONOMICAL POLARIMETRY, LIGHT SCATTERING, POLARIZED RADIATION, SEYFERT GALAXIES, SPACEBORNE ASTRONOMY, ULTRAVIOLET ASTRONOMY, APERTURES, ASTRONOMICAL MODELS, ELECTRON SCATTERING, GALACTIC RADIATION, HUBBLE SPACE TELESCOPE, ULTRAVIOLET SPECTRA
DOI:		10.1086/174395
Bibliographic Code:		1994ApJ...430..210A

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 sec x 1.4 sec., 1.0 sec., and 0.3 sec.) by the Hubble Space Telescope (HST). 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 Antonicci & Miller (1985) and Code et al. (1993). Shortward of 2700 A, the continuum polarization is constant, P is approximately 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 lambda lambda 2796, 2804 and C III lambda 1909, are seen in polarized flux Ly-alpha and C IV are not included in our spectral region). The total flux data also indicate that from approximately 2200 A to approximately 3000 A, the spectrum of NGC 1068 is strongly affected by blended Fe II emission. These polarimetry data are explained by the occulation/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 feature 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 indicated that the scattering region is spatially extended on a scale of approximatley 1 sec. Also, in the 0.3 sec aperture, and to a lesser extent in the 1.0 sec aperture, the polarization is is somewhat higher, and the position angle somewhat lower, than in 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 paricles.

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