1997A&A...322..400C

We present a unified picture of active galactic nuclei which we construct from a detailed model of line emission in the active source in NGC 4151. This source provides us with an opportunity to explore the variation of structure with luminosity in an object which, in the model, derives its unusual properties as a consequence of the angle of the accretion disc to the line of sight. The key features that emerge from the model are (i) a non-spherical broad line region (BLR); (ii) short-lived BLR clouds; (iii) a luminosity-dependent structure for the BLR; (iv) a luminosity-dependent flared accretion disc extending beyond the BLR and (v) a separate intermediate line region between the BLR and NLR. The special orientation turns out to enable us to fix many of the parameters of the model for this source. It is then natural to ask how this model would appear, in its various luminosity states, at other orientations. To make contact with observations we need to include dust obscuration over a range of angles near to the plane of the disc. We then obtain the spread of observed types of radio-quiet active nuclei and we propose an extension of the model to a unification of radio-loud active galaxies. Thus, in this scheme NGC 4151 can be regarded as a typical active nucleus, special only in its orientation. We shall find that this alleviates a number of problems with a unified picture based on a dusty molecular torus with a fixed structure (to which NGC 4151 appears as an anomaly). In the proper sense of the expression, NGC 4151 may be the exception that proves the rule.