§3 Unified model for AGN

The idea of unification for different types of active galactic nuclei came about when it was realized that projection effects must play an important role in the interpretation of some of these sources. Much effort has been expended in determining to what extent different types of AGN are simply different manifestations of the same object viewed from different angles.

The first convincing evidence that Seyfert I and Seyfert II galaxies are one and the same object was put forward by Antonucci & Miller (1985) from studies of the Seyfert II galaxy NGC 1068. They observed polarised scattered broad line emission which was as broad as the broad permitted lines seen in Seyfert I galaxies. These observations gave rise to what has become the standard model for the unification of Seyfert I and Seyfert II galaxies. There exists a ``obscuring torus'' centred about the nucleus of the galaxy which, when observed at a small angle to the axis of the torus, the active nucleus and the broad-line regions are observed. The object is classified as a Seyfert I galaxy. If the axis of the torus is observed at a large angle to the line of sight, only the narrow-line regions, located further away from the nucleus are observed.

Another unification scheme, first proposed by Barthel (1989b); Barthel (1989a), has been developed for strong radio sources. The idea behind this unification is that radio galaxies and radio quasars are the same class of object, viewed at different angles. The model suggests that there is an obscuring torus at the centre of radio galaxies; when the observer views the source within a cone of half-angle roughly $\unit{ 45 }{ \degree }$ with respect to the axis of the radio source/torus, the object is identified as a quasar. When the torus hides the active nucleus from the observer, the source is identified as a radio galaxy; when the source is viewed very near the axis of the radio source, the emission of the relativistic flow inside the jet is enhanced because of relativistic beaming and the objects are identified as blazars.

Figure I.2: Structure and mechanism of strong radio sources. A well collimated flow of relativistic material (electron--positron plasma) expands into the intergalactic and interstellar medium of a galaxy buried in the nucleus of the AGN. As the jets expand they form a cavity or cocoon made of jet material which is made of gas from the jet as it is recycled back towards the nucleus or core of the source. The termination of the jets occur on a three dimensional dumbbell structure called lobe. The brightest region in the lobes, and the whole source is called hot spot. They are the product of the collision of the jets with the interstellar or intergalactic medium as they expand. The expansion of the cocoon into the interstellar or intergalactic medium produces a bow shock. Behind this bow shock, lies a region of shocked interstellar or intergalactic medium material which is the interface between the cocoon and the external medium. The boundary between the cocoon and the shocked intergalactic medium is a contact discontinuity. This diagram was taken from Begelman & Rees (1996).

As a result of observations of powerful double radio galaxies, Scheuer (1974) developed what has become the standard model for this type of radio sources (see fig.(I.2)). There is a supermassive blackhole (Blandford, 1990; Begelman et al., 1984) located in the centre of the active galaxy and a pair of relativistic (most probably) electron-positron continuous beams, or jets, expand into the interstellar and intergalactic medium of the source. Each jet forms a very strong shock at its end and the regions where the jet interacts with the intergalactic and interstellar medium are identified in radio maps as hot spots. The shocked material from the head of the jet is recycled back towards the galaxy and forms a cavity or cocoon which is effectively a waste basket from the jet material. The cocoon is identified with the extended radio lobes of the radio source. The cocoon and the jet are assumed to be in pressure equilibrium. The expansion of the cocoon into the external medium, that is, the intergalactic or interstellar medium of the host galaxy, produces a bow shock. Behind this bow shock there is a region of shocked interstellar or intergalactic medium. The boundary between this shocked material and the cocoon has to be necessarily a contact discontinuity.