The AGN Paradigm

The following two cartoons summarize the current paradigm by which the nature of the various type of active galactic nucleus can be understood:

Direct observational evidence in support of this model comes from an HST image of the dust torus around the centre of the radio galaxy NGC4261:
Further support comes from this HST image of the Seyfert galaxy NGC 5728:

Here, ionizing ultraviolet radiation is emitted by the central source, but it is absorbed by the dust torus, and can thus only escape from the central region in a cone. This image is taken in the light of doubly ionized oxygen, and so reveals the ionization due to the escaping ultraviolet radiation.