The location of the hidden nucleus of NGC 1068

Abstract

We present HST polarization observations of NGC 1068 obtained with the Faint Object Camera in the ultraviolet (λ ∼ 2700–3700 Å), and the Wide Field Planetary Camera in the visual (λ ∼ 5000–6000 Å), at a resolution of 0.″06 and 0.″08, respectively. The UV continuum polarization is very high, peaking at ∼ 60% in the vicinity of the emission line knot 4″.5 NE of the nucleus. To a high degree of precision the polarization vectors show the centro-symmetric pattern expected from scattering from a point source. By locating the centre of symmetry of this pattern we have determined the location of the hidden nuclear source, to an accuracy of ±0.″05, which lies 0.″65 South of the emission peak and 0.″35 South of the 12.4μ peak. A pair of highly polarized clouds (P ∼ 45% in the UV) lies close to the position of the scattered nuclear source and correspond to the “twin cresent” object seen in FOC [OIII] images. The WF/PC-I polarization images contain contributions from both the optical continuum and emission lines. Generally, the large scale polarization structure is dominated by scattered [O III] emission from knot B, except at the edge of the cavity when other NLR knots contribution. However, scattered continuum from the nucleus dominates the polarization in a narrow linear region stetching from the “hidden nucleus” to knot B, suggesting that there is a component to the nuclear light which is highly collimated.