Hubble Space Telescope Imaging and Polarimetry of NGC 5128 (Centaurus A)

Abstract

We present imaging and polarimetry of NGC 5128 in the R and I bands obtained using the Hubble Space Telescope (HST) Wide Field Planetary Camera. The morphology and polarization properties of NGC 5128 at these wavelengths are dominated by the presence of the dust lane. The apparent V-band extinction ranges from 0.5 to 7 mag. The overall pattern of polarization is very regular, with the vectors oriented along the dust lane and with the degree of polarization proportional to extinction, consistent with the polarization being dominated by dichroic transmission through aligned dust grains in the foreground. After removing the effects of dust absorption, the nuclear region of the galaxy does not show a greatly perturbed morphology, and the resulting image looks qualitatively similar to those obtained from infrared observations. The location of the maximum of intrinsic emission, after correcting for extinction, 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 very close to the semi-obscured nucleus of the galaxy. No optical synchrotron emission from the jet has been detected. This sets a lower limit to the radio-to-optical spectral index of 0.55 which is consistent with the previous determinations of the spectral index for the synchrotron jet of Centaurus A.