Hubble Space Telescope imaging of the narrow-line region of NGC 4151

We have used the Planetary Camera aboard the Hubble Space Telescope to obtain high spatial resolution [O III] λ5007 and Hα λ6563 + [N II] λλ6548, 6583 images of the nucleus of NGC 4151. Almost all of the Hα λ6563 + [N II] λλ6548, 6583 emission arises from an unresolved nuclear point source. The [O III] λ5007 image resolves the narrow-line region into a number of emission-line clouds distributed in a biconical structure with apices coincident with a central point source, a projected opening angle of 75° ± 10°, and a projected axis oriented along P.A. 60°/240° ± 5°. The position angle of the [O III] λ5007 emission is aligned with the extension of the nuclear VLBI radio source. We infer from these data a geometry of the narrow-line region that places our line of sight outside the ionization cones. The simultaneous presence of a nearly unobscured UV/ optical continuum, a central pointlike broad-line region, and a biconical ionization geometry argues that the simplest version of the unified model that invokes a very optically thick molecular torus surrounding the BLR to collimate the nuclear ionizing radiation does not apply to this object. We consider several alternative collimation mechanisms that are compatible with our data as well as with column densities inferred from the UV absorption lines and from the X-ray spectrum.