Abstract
This paper reviews the properties of so called “ionization cones”—extended regions of emission-line gas apparently photoionized by oppositely directed, bi-conical beams of radiation emitted by an active nucleus. Ionization cones have been found in about 15 Seyferts, mostly of type 2. When single (one-sided) cones are seen, they generally project against the far side of the galaxy disk, suggesting that the counter cone is present but obscured by dust in the disk. So far, clear examples of cones have not been seen in radio-loud AGN's. The emission-line cone and radio axes are very tightly aligned (to ≤ 6°), indicating that the ionizing photons and radio-emitting plasma are collimated by the same, or coplanar, disks. However, the radio ejecta are much more tightly collimated than the ionizing photons. Various lines of evidence point to strong obscuration, probably by a dusty torus, at the apex of the cones in some Seyfert 2's. The origin of the collimation of the ionizing photons remains unclear—both shadowing of an isotropic UV source by a dusty torus and direct radiation from an intrinsically anisotropic emitter (such as a radiation torus) remain viable models. Cones may be seen in a few Seyfert l's; possible explanations are reviewed. The observed opening angles of the emission-line cones are, in general, expected to differ from the true opening angles of the ionizing photon cones through simple projection effects.
Published Version
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