Hubble Space Telescope infrared imaging polarimetry of Centaurus A: Implications for the unified scheme and the existence of a misdirected BL Lac nucleus

We report results from the Hubble Space Telescope (HST)/near-infrared camer a and multiobject spectrometer (NICMOS) 2 mum imaging polarimetry of the ce ntral region of Centaurus A. In the vicinity of the nucleus we observe a co mplex polarization structure, which we explain by a combination of scatteri ng of nuclear light and dichroic polarization associated with the dust lane . The scattered nuclear radiation is found in an angular region that extend s over greater than or similar to 70 degrees, and thus it does not originat e from a highly collimated beam but is associated with more omnidirectional nuclear illumination. These observations also show the presence of an unre solved, highly polarized (P = 11.1%) nuclear source whose polarization angl e theta = 148 degrees2 is perpendicular to the jet axis. We set an upper li mit of 0 " 04 (similar to0.8 pc) to its extent. The observed nuclear polari zation 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 tha n similar to1 pc. Alternatively, we might be directly seeing the infrared c ounterpart of the radio core, similar to those found in other low-luminosit y radio galaxies observed with HST. We discuss these results in the framewo rk of the FR I/BL Lac unifying model.