Artist's impression of an Active Galactic Nucleus according to the unified model
This image shows an artist's impression of an Active Galactic Nucleus (AGN) within the currently leading scenario, the so-called unified model.
An AGN consists of a supermassive black hole lying at the core of a massive galaxy and accreting the surrounding matter at extraordinary rates, thus radiating profusely across the entire electromagnetic spectrum. These sources are often so bright that they outshine their host galaxies and have been detected out to the far reaches of the observable Universe.
The unified model explains the wide variety of features discerned in different classes of AGN in terms of the anisotropic geometry of the black hole's immediate surroundings. The accreting black hole is fed via a disc, and a thick, obscuring torus of gas and dust is believed to encompass the disc and to absorb part of the radiation it emits. The torus is shown in the image.
Depending on the orientation of the torus with respect to an observer's line of sight, the view to the galactic nucleus may be obstructed to varying degrees, giving rise to differences in the observed brightness and spectra of various classes of AGN. In addition, in some cases (known as radio-loud AGN), jets of energetic particles emanating from the vicinity of the black hole are observed, whereas in other cases (known as radio-quiet AGN) jets have not been detected.