NEAR-INFRARED AND MILLIMETER POLARIMETRY OF CEN-A

We present near-infrared polarization images of the nuclear region of Cen A, and millimetre polarimetry of the nucleus. In the near-infrared the polarization vectors mainly lie along the dust lane, with the pol arization being produced by dichroic absorption of the radiation from stars embedded in the dust lane. At 2.2 mu m the nucleus shows an addi tional larger polarized component, with position angle of polarization perpendicular to the inner radio and X-ray jet of Cen A. The millimet re observations at 800 and 1100 mu m are consistent with the nucleus o f Cen A being unpolarized at these wavelengths. Modelling of the resul ts suggests that the nuclear polarization observed at 2.2 mu m is prod uced by scattering, with the scattered radiation observed through simi lar to 16 mag of extinction for a power-law central source with spectr al index a of 1.3 (F-v proportional to v(-alpha)). The central source is not observed directly in the near-infrared because of very high ext inction to it along the line of sight. Our near-infrared results do no t preclude the central source being a BL Lac type object, as several a uthors have suggested, although the zero polarization at similar to 1 mm is unexpected if the radiation at these wavelengths is dominated by non-thermal emission, as has been proposed. We present arguments whic h might explain the low polarization at these wavelengths while still allowing the central source to be a BL Lac type object. Based on our n ear-infrared model, the luminosity of the central source at 2.2 mu m i s 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 Cen A. This suggests that the jet is not scattered radiation from the cen tral source and is most likely free-free emission from gas shocked and heated by the jet of Cen A.