BVRI IMAGING POLARIMETRIC STUDIES OF THE GALAXY NGC-5128

Multi-colour imaging polarimetry of the galaxy NGC 5128 shows that pol arization is present throughout the dust lane with patterns of orienta tion which are coherent on kpc scale-lengths. In the central regions o f the dust lane, within +/-0.7 kpc of the projected position of the ga lactic nucleus, the polarization is oriented parallel to the long axis of the lane and reaches levels up to 5 per cent. At the outer extremi ties of the the dust lane, higher levels of polarization are found wit h orientations perpendicular to the axis of the dust lane. In the inte rmediate regions, null points in the polarization are observed in addi tion to an unexpected reversal of polarization orientation across the dust lane. We are unable to account for all our observations - polariz ation maps and the wavelength dependence of polarization in the dust l ane - without involving two competing polarizing mechanisms, namely, d ichroic extinction and scattering by dust grains. The polarimetric pro perties of NGC 5128 are very similar to those found in galaxies such a s M104, IC 4329A and NGC 2146, and it appears that azimuthal magnetic fields, confined to the equatorial planes of galaxies with prominent d ust lanes, are a common occurrence. The nature and origins of these ma gnetic fields are considered.