An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The radiation from AGN is believed to be a result of accretion of mass by the supermassive black hole at the centre of the host galaxy. AGN are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; their evolution as a function of cosmic time also provides constraints on models of the cosmos.
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The issue of the activity of nuclei of galaxies (AGN) was first raised by Soviet-Armenian physicist Prof. Victor Ambartsumian in the early 1950s. Although the idea concerning the activity of galactic nuclei for the first time was accepted very skeptically, after many years, as a result of the pressure of observations (the discovery of quasars, radio outbursts of galaxies, consequences of explosions in nuclei, ejection from nuclei, etc.) it did gain recognition. The concept of AGN now is widely accepted.[1]
For a long time it has been argued[2] that AGN must be powered by accretion onto massive black holes (with masses between 106 and 1010 times that of the Sun). 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, so that it can provide the observed high persistent luminosity. Central supermassive black holes are now believed to exist in the centers of most or all massive galaxies: the mass of the black hole correlates well with the velocity dispersion of the galaxy bulge (the M-sigma relation) or with bulge luminosity (e.g.[3]). Thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole.
In the standard model of AGN, cold material close to the central 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 around a supermassive black hole 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 radiates via emission lines. A large fraction of the AGN's primary output 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.
At least some accretion discs produce jets, twin highly collimated and fast outflows that emerge in opposite directions from close to the disc (the direction of the jet ejection must be determined either by the angular momentum axis of the disc or the spin axis of the black hole). The jet production mechanism and indeed the jet composition on very small scales are not known at present, as observations cannot distinguish between the various theoretical models that exist. The jets have the most obvious observational effects in the radio waveband, where Very Long Baseline Interferometry can be used to study the synchrotron radiation they emit down to sub-parsec scales. However, they radiate in all wavebands from the radio through to the gamma-ray via the synchrotron and inverse-Compton process, and so AGN with jets have a second potential source of any observed continuum radiation.
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),[4] but others 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 radiate away the energy that it has acquired in moving close to the black hole. Radiatively inefficient accretion has been used to explain the lack of strong AGN-type radiation from massive black holes in the centres of elliptical galaxies in clusters, where otherwise we might expect high accretion rates and corresponding high luminosities.[5] Radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc.
There is no single observational signature of an AGN. The list below covers some of the historically important features that have allowed systems to be identified as AGN.
It is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud. In the radio-loud objects a contribution from the jet(s) and the lobes they inflate dominates the luminosity of the AGN, at least at radio wavelengths but possibly at some or all others. Radio-quiet objects are simpler since jet and jet-related emission can be neglected.
AGN terminology is often confusing, since the distinctions between different types of AGN sometimes reflect historical differences in how objects were discovered or initially classified, rather than real physical differences.
See main article radio galaxies for discussion of the large-scale behaviour of the jets. Here only the active nuclei are discussed.
These galaxies can be broadly summarised by the following table:
| Galaxy Type | Active
Nuclei |
Emission Lines | X-rays | Excess of | Strong
Radio |
Jets | Variable | Radio
loud |
||
|---|---|---|---|---|---|---|---|---|---|---|
| Narrow | Broad | UV | Far-IR | |||||||
| Normal | no | weak | none | weak | none | none | none | none | no | no |
| Starburst | no | yes | no | some | no | yes | some | no | no | no |
| Seyfert I | yes | yes | yes | some | some | yes | few | no | yes | no |
| Seyfert II | yes | yes | no | some | some | yes | few | yes | yes | no |
| Quasar | yes | yes | yes | some | yes | yes | some | some | yes | 10% |
| Blazar | yes | no | some | yes | yes | no | yes | yes | yes | yes |
| BL Lac | yes | no | none/faint | yes | yes | no | yes | yes | yes | yes |
| OVV | yes | no | stronger than BL Lac | yes | yes | no | yes | yes | yes | yes |
| Radio galaxy | yes | some | some | some | some | yes | yes | yes | yes | yes |
Unified models of AGN unite two or more classes of objects, based on the traditional observational classifications, by proposing that they are really a single type of physical object observed under different conditions. The currently favoured unified models are 'orientation-based unified models' meaning that they propose that the apparent differences between different types of objects arise simply because of their different orientations to the observer. For an overview of these see[14] and,[15] though some details in the discussion below have emerged since these reviews were written.
At low luminosities, the objects to be unified are Seyfert galaxies. The unified models propose that in Seyfert 1s the observer has a direct view of the active nucleus. In Seyfert 2s it is observed through an obscuring structure which prevents a direct view of the optical continuum, broad-line region or (soft) X-ray emission. The key insight of orientation-dependent accretion models is that the two types of object can be the same if only certain angles to the line of sight are observed. The standard picture is of a torus of obscuring material surrounding the accretion disc. It must be large enough to obscure the broad-line region but not large enough to obscure the narrow-line region, which is seen in both classes of object. Seyfert 2s are seen through the torus. Outside the torus there is material that can scatter some of the nuclear emission into our line of sight, allowing us to see some optical and X-ray continuum and, in some cases, broad emission lines—which are strongly polarized, showing that they have been scattered and proving that some Seyfert 2s really do contain hidden Seyfert 1s. Infrared observations of the nuclei of Seyfert 2s also support this picture.
At higher luminosities, quasars take the place of Seyfert 1s, but, as already mentioned, the corresponding 'quasar 2s' are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect except through their luminous narrow-line and hard X-ray emission.
Historically work on radio-loud unification has concentrated on high-luminosity radio-loud quasars. These can be unified with narrow-line radio galaxies in a manner directly analoguous to the Seyfert 1/2 unification (but without the complication of much in the way of a reflection component: narrow-line radio galaxies show no nuclear optical continuum or reflected X-ray component, although they do occasionally show polarized broad-line emission). The large-scale radio structures of these objects provide compelling evidence that the orientation-based unified models really are true.[16][17][18] X-ray evidence, where available, supports the unified picture: radio galaxies show evidence of obscuration from a torus, while quasars do not, although care must be taken since radio-loud objects also have a soft unabsorbed jet-related component, and high resolution is necessary to separate out thermal emission from the sources' large-scale hot-gas environment.[19] At very small angles to the line of sight, relativistic beaming dominates, and we see a blazar of some variety.
However, the population of radio galaxies is completely dominated by low-luminosity, low-excitation objects. These do not show strong nuclear emission lines — broad or narrow — they have optical continua which appear to be entirely jet-related,[10] and their X-ray emission is also consistent with coming purely from a jet, with no heavily absorbed nuclear component in general.[11] These objects cannot be unified with quasars, even though they include some high-luminosity objects when looking at radio emission, since the torus can never hide the narrow-line region to the required extent, and since infrared studies show that they have no hidden nuclear component:[20] in fact there is no evidence for a torus in these objects at all. Most likely, they form a separate class in which only jet-related emission is important. At small angles to the line of sight, they will appear as BL Lac objects.[21]
For a long time, active galaxies held all the records for the highest-redshift objects known, because of their high luminosity (either in the optical or the radio): they still have a role to play in studies of the early universe, but it is now recognised that by its nature an AGN gives a highly biased picture of the 'typical' high-redshift galaxy.
More interesting is the study of the evolution of the AGN population. Most luminous classes of AGN (radio-loud and radio-quiet) seem to have been much more numerous in the early universe. This suggests (1) that massive black holes formed early on and (2) that the conditions for the formation of luminous AGN were more readily available in the early universe — for example, that there was a much higher availability of cold gas near the centre of galaxies than there is now. It also implies, of course, that many objects that were once luminous quasars are now much less luminous, or entirely quiescent. The evolution of the low-luminosity AGN population is much less well constrained because of the difficulty of detecting and observing these objects at high redshifts.
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