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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Active galactic nucleus | 2/4 | https://en.wikipedia.org/wiki/Active_galactic_nucleus | reference | science, encyclopedia | 2026-05-05T13:31:44.530837+00:00 | kb-cron |
== Models == Since the late 1960s it has been argued that an AGN must be powered by accretion of mass onto massive black holes (106 to 1010 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. 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.
=== Accretion disc ===
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.
=== Relativistic jets ===
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.
=== Radiatively inefficient AGN === There exists a class of "radiatively inefficient" solutions to the equations that govern accretion. Several theories exist, but the most widely known of these is the Advection Dominated Accretion Flow (ADAF). 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.
== Particle acceleration == AGN are a candidate source of high and ultra-high energy cosmic rays (see also Centrifugal mechanism of acceleration).
== Observational characteristics == Among the many interesting characteristics of AGNs:
very high luminosity, visible out to very high red shifts, small emitting regions, milli-parsecs in diameter, implying high energy density, strong evolution of luminosity functions, and detectable emission across the entire electromagnetic spectrum.
== Types == The observed AGNs are grouped into dozens of different sometimes overlapping classes. AGNs are classified along multiple criteria. Two AGN may be in the same group according observations at one wavelength and in different groups according observations at another wavelength. This issues are believed to reflect the current early stages of understanding AGN. Classifications based on observations have not yet been interpreted with consistent physical models. One criteria is the radio-to-optical emission ratio or radio loudness parameter,
R
=
L
5
G
H
z
L
B
,
{\displaystyle R={\frac {L_{5GHz}}{L_{B}}},}
where
L
5
G
H
z
{\displaystyle L_{5GHz}}
is the luminosity at the 5 GHz radio band and
L
B
{\displaystyle L_{B}}
is the optical luminosity. The AGN with
R
≫
10
{\displaystyle R\gg 10}
and
L
5
G
H
z
>
10
25
Jy
{\displaystyle L_{5GHz}>10^{25}{\textrm {Jy}}}
are called radio loud, otherwise they are radio quiet. This ratio is suspect in cases where the optical emission may be obscured by dust and direct star light along the line to the AGN. Alternatively this split can be defined as a radio luminosity cutoff at a fixed frequency, e.g.
L
1.4
G
H
z
>
10
24
W
/
Hz
.
{\displaystyle L_{1.4GHz}>10^{24}{\textrm {W}}/{\textrm {Hz}}.}
A second criteria is the existence of broad emission lines in the optical spectrum: type-1 has broad lines but type-2 does not.
=== Radio-quiet AGN === When the galaxy associated with an AGN is optically resolvable they are called Seyfert galaxies. These are classed as type-1 or type-2 according to the existence of broad emission lines.