Hubble Space Telescope Infrared Imaging Polarimetry of Centaurus A: Implications for the Unified Scheme and the Existence of a Misdirected BL Lacertae Nucleus

Abstract

We report results from the Hubble Space Telescope (HST)/near-infrared camera and multiobject spectrometer (NICMOS) 2 μm 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 that extends over >~70°, and thus it does not originate from a highly collimated beam but is associated with more omnidirectional nuclear illumination. These observations also show the presence of an unresolved, highly polarized (P=11.1%) nuclear source whose polarization angle θ=148.2d 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 FR I/BL Lac unifying model. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555 and by STScI grant GO-3594.01-91A.