Near- and mid-infrared imaging polarimetry of NGC 1068

We present the results of a series of observations of the near- and mid-infrared polarization properties of the type 2 Seyfert galaxy NGC 1068. Our data agree well with previously published results in showing the need for a separate polarization mechanism in the near-infrared apart from scattering. We find that the predictions of a simple model in which this component arises through absorptive dichroism caused by aligned grains within the extended warm (∼ 400 K) dust fit the data reasonably if the obscured background source is itself the result of dust emission (at temperature >1000 K). By considering the change of polarization with wavelength we show that the extinction to this hot dust region is in the range AV=20-40. Consideration of the observed data then leads us to the conclusion that, if viewed face-on, NGC 1068 would have a strong near-infrared excess similar to type 1 Seyfert galaxies. Comparison with other independent measures of the extinction to the active nucleus itself leads us to the conclusion that the hot dust must provide screening equivalent to at least AV=40, and possibly much higher. We speculate that this component alone may be the `classical' torus discussed in terms of the unified model, and the more extensive mid-infrared emission may arise from circumnuclear molecular cloud material, and dust in the ionization cones.

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