TAURUS observations of the [O III] cone in NGC 1068

Abstract

TAURUS observations of the [O III] λ5007 emission from NGC 1068 show that most of the high-excitation line emission originates in an edge-brightened cone. The morphology, excitation, and kinematic properties of this [O III] cone all imply that the line-emitting gas is photoionized by a highly anisotropic nuclear radiation field. The edge-brightening makes possible an accurate determination of the cone parameters, and hence the parameters of the collimated nuclear radiation field; we derive a PA of |$31^\circ \pm 3^\circ$| and a projected opening angle of |$40^\circ \pm 3^\circ$|⁠. It seems unlikely that the edge-brightening of the [O III] cone is intrinsic to the mechanism by which the radiation field is initially collimated. We suggest that the edge-brightening might be due either to the presence of relatively dense gas along the cone axis, associated with the radio components, or to a density enhancement along the cone edges, caused by a lateral expansion of the ionized material within the cone. The [O III] cone is closely aligned with the kpc-scale radio jet/lobe |$(\text{PA}=33^\circ)$|⁠. Furthermore, the [O III] and radio emission both show a side-to-side asymmetry in the same sense. This suggests a model for the collimation mechanism in which the radio jet clears a path through obscuring material surrounding the galactic nucleus, through which the ionizing radiation can then escape. However, this picture is complicated by the observation that the axis of the UV cone derived from spectropolarimetry of the optical nucleus lies in |$\text{PA}=2^\circ$|⁠, misaligned with the PA of the large-scale radio and [O III] structures, and by the fact that the subarcsecond radio structure appears to bend between these two PAs. We speculate that different collimation mechanisms may be relevant on different size-scales.