A ONE-SIDED IONIZATION CONE IN THE SEYFERT 2 GALAXY NGC 5643

摘要

We present high-resolution (0.1″) observations of the Seyfert 2 galaxy NGC 5643 taken with the WFPC2 on board the Hubble Space Telescope. Images taken in the light of [O III] λ5007 and Hα reveal a halo of emission centered on the nucleus and high-excitation extranuclear emission which extends eastward for at least 1.8 kpc. An excitation map of [O III]/Hα reveals the high-excitation gas to be distributed within a well-defined V-shaped structure, presumably the projection of a three-dimensional cone. The apex of the cone is coincident with the peak of the red continuum emission and shows a low [O III]/Hα ratio, which is probably a result of foreground reddening. There is evidence that the fine-scale structure of the ionized gas is related to the radio ejecta, which share the same axis as the cone. We detect a dust lane straddling the nucleus and oriented perpendicular to the radio axis, and an unusually blue region of ～ 90 pc extent to the east of the nucleus. Our data strongly support the unified model for Seyfert galaxies, in which an optically thick structure perpendicular to the radio axis blocks the nucleus from direct view and allows the optical and ultraviolet radiation to escape preferentially along and around this axis.