HUBBLE-SPACE-TELESCOPE IMAGING AND POLARIMETRY OF NGC-5128 (CENTAURUS-A)

We present imaging and polarimetry of NGC 5128 in the R and I bands ob tained using the Hubble Space Telescope (HST) Wide Field Planetary Cam era. The morphology and polarization properties of NGC 5128 at these w avelengths are dominated by the presence of the dust lane. The apparen t V-band extinction ranges from 0.5 to 7 mag. The overall pattern of p olarization is very regular, with the vectors oriented along the dust lane and with the degree of polarization proportional to extinction, c onsistent with the polarization being dominated by dichroic transmissi on through aligned dust grains in the foreground. After removing the e ffects of dust absorption, the nuclear region of the galaxy does not s how a greatly perturbed morphology, and the resulting image looks qual itatively similar to those obtained from infrared observations. The lo cation of the maximum of intrinsic emission, after correcting for exti nction, is spatially coincident with the K-band peak as observed from the ground and also coincides with the maximum of obscuration and the highest polarization region. A compact, highly polarized emission knot seen at that location might be identified with a scattering cloud ver y close to the semiobscured nucleus af the galaxy. No optical synchrot ron emission from the jet has been detected. This sets a lower limit t o the radio-to-optical spectral index of similar to 0.55 which is cons istent with the previous determinations of the spectral index for the synchrotron jet of Centaurus A.