<P> For a long time it has been argued that an AGN must be powered by accretion of mass onto massive black holes (10 to 10 times the Solar mass). AGN are both compact and persistently extremely luminous . Accretion can potentially give very efficient conversion of potential and kinetic energy to radiation, and a massive black hole has a high Eddington luminosity, and as a result, it can provide the observed high persistent luminosity . Supermassive black holes are now believed to exist in the centres of most if not all massive galaxies since the mass of the black hole correlates well with the velocity dispersion of the galactic bulge (the M - sigma relation) or with bulge luminosity . Thus AGN - like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole . </P> <P> In the standard model of AGN, cold material close to a black hole forms an accretion disc . Dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up . The expected spectrum of an accretion disc peaks in the optical - ultraviolet waveband; in addition, a corona of hot material forms above the accretion disc and can inverse - Compton scatter photons up to X-ray energies . The radiation from the accretion disc excites cold atomic material close to the black hole and this in turn radiates at particular emission lines . A large fraction of the AGN's radiation may be obscured by interstellar gas and dust close to the accretion disc, but (in a steady - state situation) this will be re-radiated at some other waveband, most likely the infrared . </P> <P> Some accretion discs produce jets of twin, highly collimated, and fast outflows that emerge in opposite directions from close to the disc . The direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole . The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low . The jets have their most obvious observational effects in the radio waveband, where very - long - baseline interferometry can be used to study the synchrotron radiation they emit at resolutions of sub-parsec scales . However, they radiate in all wavebands from the radio through to the gamma - ray range via the synchrotron and the inverse - Compton scattering process, and so AGN jets are a second potential source of any observed continuum radiation . </P> <P> There exists a class of' radiatively inefficient' solutions to the equations that govern accretion . The most widely known of these is the Advection Dominated Accretion Flow (ADAF), but other theories exist . In this type of accretion, which is important for accretion rates well below the Eddington limit, the accreting matter does not form a thin disc and consequently does not efficiently radiate away the energy that it acquired as it moved close to the black hole . Radiatively inefficient accretion has been used to explain the lack of strong AGN - type radiation from massive black holes at the centres of elliptical galaxies in clusters, where otherwise we might expect high accretion rates and correspondingly high luminosities . Radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc . </P>

Select the combination of components responsible for active galactic nuclei in current models