HST infrared imaging polarimetry of Centaurus A: implications for the unified scheme and the existence of a mis-directed BL Lac nucleus

Abstract

We report results from HST/NICMOS 2 micron imaging polarimetry of the central region of Centaurus A. In the vicinity of the nucleus we observe a complex polarization structure which we explain by a combination of scattering of nuclear light and dichroic polarization associated with the dust lane. The scattered nuclear radiation is found in an angular region which extends over ~ 70 degrees and thus it does not originate from a highly collimated beam, but is associated with more omni-directional nuclear illumination. These observations also show the presence of an unresolved, highly polarized (P = 11.1 %) nuclear source whose polarization angle PA = 148.2 degrees is perpendicular to the jet axis. We set an upper limit of 0.04'' (~0.8 pc) to its extent. The observed nuclear polarization is naturally accounted for if we are observing scattered light from an otherwise obscured nucleus provided that both the scattering region and the occulting torus are extremely compact, with an outer radius of less than ~ 1 pc. Alternatively, we might be directly seeing the infrared counterpart of the radio core, similar to those found in other low luminosity radio-galaxies observed with HST. We discuss these results in the framework of the FRI / BL Lac unifying model.