HST IMAGING POLARIMETRY OF THE SEYFERT-2 GALAXY NGC-5728

We present the results of HST imaging polarimetry of NGC 5728 taken wi th the COSTAR corrected Faint Object Camera in the ultraviolet (3000-3 700 Angstrom) and covering the region interior to its well-known nucle ar ring. The total intensity image is dominated by a bright extended c ore, 1'' x 0''.7 in size, which is resolved into a large number of com pact knots which are coincident with the bright inner region of the Ea stern emission-line cone found by previous observers. A barlike struct ure extends from this core for over 3'' toward the east along P.A. + 7 5 degrees to the star formation ring. On the large scale the polarizat ion vectors are predominantly aligned with the major axis of the kpc s cale bar of NGC 5728 and the polarization is dominated by dichroic tra nsmission through dust grains. Significant deviations from this unifor m polarization pattern are observed in the innermost regions. Subtract ion of the dichroic polarization reveals a centro-symmetric polarizati on pattern, due to scattering of nuclear light, except in the region o f the inner bar. As in the large-scale bar, the polarization vectors i n the inner bar are parallel with its major axis, and imply the presen ce of a magnetic held which is similarly aligned. The associated polar ized flux distribution appears as an approximately double-lobed struct ure, extending for at least 1.2 kpc in a sector from P.A. similar to 5 3 degrees to similar to 147 degrees in which the bright core stands ou t prominently, even in the knots, showing it is a region of enhanced s cattering. The angular distribution of the polarized flux is substanti ally wider than the emission lines cones which have an opening angle b etween 55 degrees-65 degrees. A similar result has been found previous ly in NGC 1068. This can be explained by the presence in the obscuring torus of a transition region which is transparent to the UV light whi le still blocking the ionizing flux. Alternatively, the line emitting material does not hh the region illuminated by the nucleus. In each ca se, the scattered light is a better tracer of the geometry of the nucl ear radiation field. We have used the center of symmetry of the centro -symmetric polarization pattern to determine the location of the nucle us, which we find to be at R.A. 14(h)42(m)23(s).91, decl. -17 degrees 42'10''.9, and which lies in the dust lane dividing the emission line bi-cone. As expected from the unified schemes, this result confirms th at the nucleus of NGC 5728 is hidden from our direct view. A conservat ive upper limit of 1.9% to the FOC/COSTAR instrumental polarization ca n be set comparing these results with ground based observations.