Imaging Polarimetry of the Seyfert 2 Active Galactic Nucleus in NGC1068

Stuart Young¹, J.H Hough¹, Chris Packham², Antonio Chrysostomou^1,3, Jeremy Bailey⁴, Stuart Lumsden⁴

1 : Dept. of Pjysical Sciences, University of Hertfordshire, Hatfield, HERTS AL10 9AB, UK

2 : Isaac Newton Group, Apartado 321, 38780 Santa Cruz de la Palma, Islas Canarias, Spain

3 : Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, Hawaii 96720, USA

4 : Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia

To account for the mid- to far-infrared emission of NGC 1068, a substantial amount of dust is required in a compact region about the nucleus.  The spectral energy distribution (SED) has been successfully modelled using a torus-type geometry.  Different groups, however, infer different sizes for the torus, from ~ 10 pc (Pier & Krolik, 1993, ApJ, 418, 673) to ~ 200 pc (Efstathiou, Hough & Young, 1995, MNRAS, 277, 1134).  At optical wavelengths the emission line/scattering region for NGC 1068 is one-sided owing to the extinction through the host galaxy, however, at near-IR wavelengths the true bi-conical structure is apparent (Packham et al., 1997, 288, 375).  This leads to a more direct observational test for the torus size, as a large torus will be visible in silhouette against the counter-cone.

It was with this aim that imaging polarimetry of NGC 1068 was obtained using UKIRT with IRCAM3+IRPOL2 during the commissioning of the polarimeter, built by the University of Hertfordshire, in August 1995.  These observations proved to be completely consistent with the large torus model, and additionally were also consistent with modelling of the polarized flux spectrum (Miller, Goodrich & Mathews, 1991, ApJ, 378, 47; Young et al., 1995, MNRAS, 272, 513).  Derived parameters were a position angle on the sky of 32 +/- 3 degrees, an inclination of the torus polar axis to the line of sight of 42 +/- 4 degrees and a minimum diameter for the torus of 200 pc.
 
 

FIGURE 1 : The H-band polarized flux intensity of NGC 1068. The contours represent the fainter features of the image, whilst the false colours emphasise the detail in the central nuclear region. North is to the top and east to the left. 

The recent upgrades at UKIRT, with significant improvement in image quality and stability promised a better determination of the torus parameters, and so NGC 1068 was observed again on 1997 October 1.  The resultant H-band (1.6 microns) polarized flux image is displayed in Fig. 1.  The contours represent the fainter features of the image and the false-colour image is of the brighter features.  Several new features are visible in this image (resolution 0.25 arcsec), the most striking of which is that the scattering cones have been resolved into a marked X-shaped pattern.

The north pointing component of the forward cone seems to be spatially associated with the narrow-line emission features observed with the HST, with the resolved knot of polarized flux immediately to the north of the nucleus (in contour) being co-spatial with the knots D and E of the Evans et al (1991, ApJ, 369, L27) emission line maps.  The polarized flux extends approximately 3 arcsec to the north, which is equal to the diffuse filaments in the H-alpha image of Capetti, Axon & Macchetto (1997, ApJ, 487, 560: middle image of their Fig 12; be aware that their scale bars for the right two images of Fig 12 are incorrect and should be 3 and 1 arcsec as in Fig 4 Plate 11).  Any polarized flux associated with the emission line knots closer to the nucleus are unresolved from the nucleus in Fig 1.  The dark lane to the north-east is associated with the radio jet, which is also coincident with the dark lane through the emission line region (Capetti, Macchetto & Lattanzi, 1997, ApJ, 476, L67).  The bulk of the scattered polarized flux follows the centre line of the torus axis, and is probably from the large but more diffusely filled scattering cone; it may also result from the interception of the radiation cone and the host galaxy disk.
 
 

FIGURE 2 : A false colur image of the fainter regions of the H-band polarized intensity image of NGC 1068 (left) and a repeat of Figure 1 (right). The named regions are features of the polarized intensity related to physical structures of the active nucleus, as discussed in the text.

The counter-cone, to the south-west, also shows a prominent dark lane pointing away from the nucleus, co-spatial with the inner counter-jet, however, emission line images for the counter regions do not exist to compare features.  The absorption owing to the torus is clearly evident on both arms of the counter-cone polarized flux, and a reliminary
geometrical analysis along the lines of Young et al. (1996) result in similar values for the position angle of the torus axis on the sky and the inclination to the line of sight.  The better image quality compared to the August 1995 observations results in a better estimate of the minimum size of the torus at 180 pc, close to that calculated from the SED model fits (Efstathiou, Hough & Young 1995).

The nuclear region shows a D-shape structure with an extension along the postulated torus axis.  The D-shape seems to consist of an unresolved component comprising of a dichroic view of the infrared emitting region through the torus, with a possible scattering component as well. For approximately 180 degrees around the nucleus to the north-east there is a fainter C-shaped arc, the shape of which suggests it may result from scattering off the top surface of the torus, with any scattering from the near-side forward funnel being attenuated by the bulk of the torus.  if the torus is scattering radiation directly from the near-IR emission region then the top-side of the torus, within ~ 40 pc, must have a convex shape, again in excellent agreement with the torus models of Efstathiou, Hough & Young (1995).

The UKIRT upgrades have revealed a wealth of new information for NGC 1068, through imaging polarimetry.  Preliminary analysis of this new data strengthens the unified theory, and provides additional support for our scattering and radiation transfer models for NGC 1068.

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