Is the polarization of NGC1068 evidence for a non-thermal source?

Abstract

NGC1068 is one of the brightest galaxies included by Seyfert¹ in his list of extragalactic objects having compact, luminous nuclei within which broad, high-excitation emission lines occur. It has been the subject of intensive studies at UV^2,3, optical^3–6, IR^7–12 and radio wavelengths^13–15. Unresolved questions concern the nature and relationship of the sources of the excess flux seen in the UV and IR, their connection with the collimated jets apparent in high-resolution radio maps and their association with the extended region responsible for the broad emission lines. A further question is the location of any dust and its role in modifying the optical and UV spectrum. We report here observations with two high-resolution optical spectro-polarimeters which throw new light on these questions. From detailed structure found in the linear polarization spectrum of the nucleus we conclude that dilution by starlight modifies the polarization to an extent not previously appreciated. In fact, the polarization of the non-stellar flux in the optical and near IR is approximately independent of wavelength (as expected for synchrotron emission or electron scattering) with a direction orthogonal to that of the radio jets; such an arrangement is reminiscent of certain quasars and radio galaxies^16,17.