BVRI imaging polarimetric studies of the galaxy NGC 5128

Abstract

Multi-colour imaging polarimetry of the galaxy NGC 5128 shows that polarization is present throughout the dust lane with patterns of orientation which are coherent on kpc scale-lengths. In the central regions of the dust lane, within ±0.7 kpc of the projected position of the galactic nucleus, the polarization is oriented parallel to the long axis of the lane and reaches levels up to 5 per cent. At the outer extremities of the the dust lane, higher levels of polarization are found with orientations perpendicular to the axis of the dust lane. In the intermediate regions, null points in the polarization are observed in addition to an unexpected reversal of polarization orientation across the dust lane. We are unable to account for all our observations — polarization maps and the wavelength dependence of polarization in the dust lane — without involving two competing polarizing mechanisms, namely, dichroic extinction and scattering by dust grains. The polarimetric properties of NGC 5128 are very similar to those found in galaxies such as M104, IC 4329A and NGC 2146, and it appears that azimuthal magnetic fields, confined to the equatorial planes of galaxies with prominent dust lanes, are a common occurrence. The nature and origins of these magnetic fields are considered.