THE NATURE OF THE OPTICAL FILAMENTS IN CENTAURUS-A EVIDENCE FOR A BEAMED IONIZING CONTINUUM

We present new observation, including both narrow-band imaging and long-slit spectroscopy, of the optical filaments in Centaurus A. The line-emitting filaments which compose part of the well-known 'optical jet' are complex and highly structured at the limit of ground-based resolution. Variations in the surface brightness, line intensity ratios and the velocity field occur on spatial scales approximately 100 pc. We find no evidence of [O III] lambda-5007 emission associated with either the 'inner jet' or a possible 'counter jet'. The emission line spectra of the filaments are very similar in character to those of the spatially extended nebulosities in more distant radio galaxies and radio quasars. We conclude from an analysis of the line intensity ratios that the filaments are predominantly photoionized by the radiation field of a nuclear continuum source which is hidden from our direct view either by obscuration, intrinsic anisotropy, or both. Spatial variations in the line intensity ratios can be explained by local differences in the distribution of densities among an ensemble of clouds. The radiation field which is required to account for the ionization state of the emitting gas is such that the nuclear continuum source seen by then has an intensity in the 2-6 keV X-ray band which is 200 times greater than that which is observed directly from Earth. This 'X-ray deficit' can be explained if the nuclear continuum is relativistically beamed in the direction of the optical jet. We show that, if this is the case, then Centaurus A has a beam power similar to that of BL Lac.