Near-infrared and millimetre polarimetry of CEN A

Abstract

We present near-infrared polarization images of the nuclear region of CenA, and millimetre polarimetry of the nucleus. In the near-infrared the polarization vectors mainly lie along the dust lane, with the polarization being produced by dichroic absorption of the radiation from stars embedded in the dust lane. At 2.2 μm the nucleus shows an additional larger polarized component, with position angle of polarization perpendicular to the inner radio and X-ray jet of CenA. The millimetre observations at 800 and 1100 μm are consistent with the nucleus of CenA being unpolarized at these wavelengths. Modelling of the results suggests that the nuclear polarization observed at 2.2 μm is produced by scattering, with the scattered radiation observed through ~16 mag of extinction for a power-law central source with spectral index alpha of 1.3 (F_v~v^-alpha). The central source is not observed directly in the near-infrared because of very high extinction to it along the line of sight. Our near-infrared results do not preclude the central source being a BL Lac type object, as several authors have suggested, although the zero polarization at ~1 mm is unexpected if the radiation at these wavelengths is dominated by non-thermal emission, as has been proposed. We present arguments which might explain the low polarization at these wavelengths while still allowing the central source to be a BL Lac type object. Based on our near-infrared model, the luminosity of the central source at 2.2 μm is weaker by a factor of 100 compared to that of BL Lac, approximately the same factor as at X-ray wavelengths. There is no evidence for any additional polarization associated with the `blue' infrared jet of CenA. This suggests that the jet is not scattered radiation from the central source and is most likely free-free emission from gas shocked and heated by the jet of CenA.