Since the discovery of quasars in the 1960s, many objects with similar characteristics have been discovered successively, all of which are alien systems, collectively referred to as active galactic nuclei.
Common observation characteristics mainly include:
Bright dense nuclear area;
The spectrum has a very high red shift, indicating that the distance is far in the cosmological scale, and the luminosity is very high, far higher than that of ordinary galaxies.
It has rapid light change, and the light change time scale varies from hours to days, indicating that its scale only accounts for a small part of the whole galaxy.
There are very wide emission lines in the spectrum;
Non thermal radiation spectrum;
With optical or radio jet phenomenon.
There are many kinds of active galactic nuclei discovered in recent decades, including Seyfert galaxies, quasars, radio galaxies, and BL type objects in the constellation Scorpius, and the observation characteristics of different kinds are mixed. For a long time, people have been puzzled about their mechanism and evolution, and have invested a lot of human and material resources in research, making AGN one of the hottest and most active research fields in astronomy since the 1990s. It is widely accepted that AGN is composed of super heavy black holes and accretion disks. Based on theoretical and observational research, people have established the standard model of AGN, that is, the center is a super heavy black hole, and the surrounding matter is gravitationally dropped, forming an accretion disk around the black hole. Because of the dissipation, the gas is heated to a very high temperature, and gradually falls to the center of the black hole, and forms a jet along the normal direction of the accretion disk. The observational characteristics of AGN mainly depend on the characteristics of the central black hole, accretion disk and the line of sight direction.
In February 2024, using the early scientific data of the Dark Energy Spectral Survey (DESI), researchers from the National Astronomical Observatory of the Chinese Academy of Sciences and other institutions found 56 "face changing" active galactic nuclei and 44 "face changing" active galactic nucleus candidates. Relevant research results were published online in the Astrophysical Journal Supplement. [3]
In order to make the meaning of AGN more clear, the following observation characteristics can be used to define it:
1. Comparison Normal galaxy Brighter dense nuclei; 
Active galactic nucleus 4. The intensity, polarization and spectral shape of continuous spectrum and emission line change with time;
5. Compared with normal galaxies High-energy photon (e.g X-ray ， Gamma ray And so on). Nuclei with all or part of the above characteristics are defined as active galactic nuclei. a quasar With all the above characteristics, it is the most active active galactic nucleus. Scorpion tiger celestial body （ BL Lac Objects ）It has all the characteristics except the third one, so it is also a typical active galactic nucleus. Other active galactic nuclei have the third observation feature. so Spectral line The nature is the most critical identification of AGN. Typical active galactic nuclei have the following characteristics:
Photometry and general Galactic nucleus There is little difference, radio The luminosity is about 20~2 × 10 of ordinary galaxies six Times, and a quasar Up to 6 × 10 six Times. 2. Fast light change
The light variation period is generally a few days to a year or so, and even some active galactic nuclei Photoperiod Only a few hours. 3. Non thermal continuous radiation, high Polarized radiation Normal galaxy The radiation of is Blackbody radiation The maximum value is in the optical band, while the spectrum of AGN is complex, maximum value In the far infrared band (non Thermal radiation).
4. The radio band has special forms
Jet, bright core and irregular shape are often observed in radio band.
5. Intense radiation
The emission line originates from the ionization of the surrounding environment by the high-energy radiation in the central area, and usually behaves as a metal line in the spectrum.
6. Its host galaxy is relatively dim
AGN luminosity is usually 10 eleven To 10 fifteen The lower limit of its mass can be calculated from the luminosity. This lower limit is called in astronomy“ Eddington threshold ”。 It means that for a stable radiation source, the gravity caused by the mass cannot be less than the outward pressure of its radiation, otherwise, the gravity will not be enough to maintain the concentration of its own matter and will be "blown away". The high luminosity of AGN shows that they have a great mass, which should be between 1 million and 10 billion Solar mass Otherwise, the pressure equivalent to its great luminosity will blow all the substances away. The most active Galactic nucleus Should belong to a quasar It should also have the maximum mass. From the light variation period of active galactic nuclei and quasars, it can be seen that they have only a very small scale. The comparison of mass and scale shows that there are Giant black hole Exists. The observation characteristics of various active galactic nuclei depend on the observation methods used, rather than the active galactic nuclei themselves. Therefore, the characteristics of various active galactic nuclei are often mixed together. Nevertheless, they can generally be divided into the following categories:
Quasar (QSO): It has a very large red shift, high luminosity, emission lines in the spectrum, power law spectrum in the visible light band, most of which have X-ray radiation, and a few have very strong radio radiation. Among them, the ones with strong radio radiation are called radio cloud quasars, the ones with weak radio radiation are called radio quiet quasars (QSOs), and some people use Quasar meters to measure radio noise quasars.
Seyfert galaxies: the earliest confirmed active galactic nuclei. It is characterized by high brightness of the nucleus, strong high ionization emission lines, wide spectral lines, strong and variable X-ray, strong infrared radiation, most of which are spiral galaxies, and also irregular galaxies. According to the width and shape of emission lines, they can be divided into Type I and Type II. Type I Seiffert galaxies have wide emission lines, while Type II only has narrow emission lines. It can also be further divided into 1.5, 1.8, 1.9 and other types. [2]
Narrow line X-ray galaxy (NLXG): It has a high ionization emission line, similar to the Seiffert galaxy, but with low luminosity. Seyfert galaxy whose spectrum is thought to be extinguished by the dust in the galaxy.
Low ionization galactic nucleus (LINER): The nucleus has a relatively low luminosity, and has a low ionization nuclear radiation area. Sometimes, it is found to be a low luminosity type 2 Seiffert galaxy.
BL Lac Objects: The galactic nucleus is very bright, and the short time exposure is similar to that of stars. The luminosity has a rapid change, the radio radiation has a strong polarization, and there is no absorption line or emission line in the spectrum, so its red shift can only be inferred from the spectrum of the host galaxy.
Optical Cataclysmic Quasars (OVVs): The luminosity has a rapid change and is often an intense radio source. Together with the BL type objects in Scorpio, it is called Blazar.
Radio Galaxy: It has very strong radio radiation, most of which have two radiation sources, called dual source radio galaxy. Usually elliptical galaxies. According to the width of emission lines, they can be divided into wide line radio galaxies (BLRG) and narrow line radio galaxies (NLRG).
Starburst Galaxies: with huge star forming regions, infrared luminosity is higher than visible luminosity, and most of them are spiral galaxies. It belongs to active galaxies, but its relationship with active galactic nuclei is still uncertain.
In addition, there are Markarian Objects, Zwicky Galaxies, N Galaxies, high polarization quasars (HPQ), low luminosity active galactic nuclei (MAGN), hot stars (Warmer), ultra bright far infrared galaxies, etc.
According to the radiation of radio band, it can also be conveniently divided into radio quiet active galactic nuclei and radio noise active galactic nuclei. Among them, radio quiet active galactic nuclei include: low ionization nuclei, Siver galaxies, and some quasars; radio noise active galactic nuclei include radio noise quasars, flare variants (including BL type objects in Scorpius and optical cataclysmic quasars), radio galaxies, and so on.
The black hole accretion disk model is widely recognized as the standard model of active galactic nuclei. In this model, the central engine of AGN is a supermassive black hole. Under the action of gravity, the gas around the black hole falls towards the black hole. With angular momentum, matter forms an accretion disk around the black hole. The accretion disk has a dissipative effect, and the gas will be heated to a very high temperature. At the same time, it will continuously lose angular momentum and gradually fall to the center of the black hole. At the same time, it will release huge gravitational energy and radiate in the form of electromagnetic waves, mainly non thermal radiation. When the accretion rate of a black hole is very high, far exceeding that of other parts of the galaxy, it will appear as an active galactic nucleus. As the material around the black hole gradually depletes, the luminosity of the core decreases, and active galaxies evolve into normal galaxies. The magnetic field plays an important role in the formation of high-speed jet along the normal direction at both ends of the accretion disk. When the angle between the jet direction and the observer's line of sight is very small, the phenomenon of apparent superluminal velocity can also be observed.
The unified model of active galactic nuclei attempts to describe two or more types of active galactic nuclei with one model. Different types of active galactic nuclei only have different observation perspectives. The radio quiet active galactic nuclei and radio loud active galactic nuclei have their own unified models: the radio weak unified model and the radio strong unified model. The weak unified radio wave model believes that the Xifo type I galaxy is the active galactic nucleus itself with lower luminosity observed directly, while the Xifo type II galaxy is blocked by the shadowing ring around the accretion disk in the line of sight direction. If the luminosity of active galactic nuclei is high, the direct observation is not the Siver I galaxy, but quasars. The unified model of radio wave intensity mainly focuses on radio wave strong quasars with high luminosity. They can be unified with narrow ray radio wave galaxies in a way similar to the weak unified model of radio wave, that is, radio wave galaxies are blocked by a masking ring, while quasars do not. If the angle between the line of sight and the jet is very small, BL type celestial quasars (blazar) in Scorpius will be observed.
