ACTIVE GALACTIC NUCLEI

Active galaxies appear in a wide variety of guises, with observed phenomena covering vast ranges in luminosity and spatial scale. This diversity has hindered attempts to understand the nature of the fundamental energy source. In recent years, however, it has emerged that at least some of the radiation produced by active galactic nuclei (AGN) is emitted anisotropically. This has led to the development of "unified schemes" which seek to relate apparently differently classes of AGN. In one model an active nucleus is largely enclosed by a thick torus of obscuring material. The strong non-stellar continuum and broad emission lines characteristic of Seyfert 1 galaxies are seen when the object is oriented so that we have an unobstructed view of the nucleus but when the obscuring disk is edge-on, the nucleus is hidden and a Seyfert 2 galaxy is seen, with narrow emission lines and weak non-stellar continuum.

As a test of these ideas, the consortium called the "Lovers of Active Galaxies" observed the circum-nuclear region of the Seyfert 1 galaxy NGC 4151, using both the WHT and INT as part of and International Time Programme, looking for evidence of effects on the surrounding interstellar medium of ionizing radiation emitted by the nucleus. Like many Seyferts, NGC 4151 contains an extra-nuclear emission line region (ENLR) which is elongated over several kiloparsecs. The LAG consortium find that the ENLR emission line spectrum is characteristic of photoionization by a AGN continuum source and that the ionizing radiation field is very probably anisotropic.. Furthermore, the ionising parameter decreases with distance from the nucleus, consistent with geometrical dilution of the radiation of a central source. It is thought that ionising radiation from the nucleus could be being emitted into a broad cone whose axis is inclinned at a large angle to the plane of the galactic disk (by about 70 degrees) and that the ENLR is that segment of the disk which is illuminated by the cone. An elongated nuclear radio source is obsserved at an angle to the ENLR: the ejection axis of the radio-emitting plasma could conincide with the axis of the cone. This geometry is consistent with the fact that NGC 4151 is a Seyfert 1, since our line of sight would fall within the cone, giving a direct view of the nucleus.