The active nuclei of galaxies are the nuclei in which processes occur, accompanied by the release of a large amount of energy, not explained by the activity of individual stars and gas-dust complexes in them [1] .
The observed signs of core activity and forms of energy release may be different. The most common manifestations of activity are [2] :
- Emissions of jets of gas or fast particles from nuclei.
- High power of radio emission associated with the release of high-energy electrons from the nucleus emitting in a magnetic field (synchrotron radiation mechanism).
- The rapid movement of gas at speeds of a thousand kilometers per second, which leads to a strong broadening of the emission lines in the spectrum of the nucleus due to the Doppler effect .
- High-power radiation in the short-wavelength (optical, ultraviolet, and X-ray) spectral regions, concentrated in a very small region less than a light year in size. Its spectrum is not similar to the spectrum of an absolutely black body and has a power-law form (F v ~ v n , where v ≈ 0.6–0.8). Radiation is usually variable without a clearly defined period; the characteristic time of a noticeable change in luminosity is from several years to several days or even hours.
An active galaxy is a galaxy with an active core. Such galaxies are divided into: Seyfert , radio galaxies , lacertids and quasars . Currently, it is believed that in the center of an active galaxy is a massive compact object, most likely a black hole , which is the reason for the increased radiation intensity, especially in the x-ray range. A relativistic jet (jet) is usually pulled out from the nuclei of such galaxies. A distinctive feature of many active galaxies is the variable (from several days to several hours ) X-rays . It is believed that quasars , Seyfert galaxies , radio galaxies and blazars are one and the same, but visible from Earth from different points of view [3] . There is evidence that a rotating galaxy becomes active periodically, that is, activity is not a property of the galaxy, but its state.
Content
AGN Models
At the moment, it is not known for certain what causes unusual behavior of active nuclei. The following versions are discussed:
- Nuclear activity is associated with supernova explosions . In this case, a supernova burst can become the starting mechanism that releases the energy stored in the entire region of the nucleus. Supernova explosions regularly occurring in the nucleus can explain the observed energetics of nuclei. But some phenomena observed in radio galaxies (emissions of matter in the form of jets of relativistic plasma), which speak of the ordered structure of the magnetic field of the nucleus, cannot be explained.
- Core activity is created by a massive star-like object with a strong magnetic field . There is an analogy with pulsars . The main problem here, as you can understand, is the object itself.
- The activity of a core with a supermassive black hole (from 10 6 to 10 9 solar masses) is the most widely accepted theory to date.
Accretion Disc
- Main article: Accretion Disc
In the standard AGN model, the accretion disk (AB) forms a substance located near the central black hole (BH). In the absence of friction, the balance of gravity created by the mass of the central body and centrifugal force leads to Kepler rotation. In this case, the angular velocity of the substance decreases with distance from the center (differential rotation). Accretion discs have high gas pressure. The differential rotation of the gas generates friction, which disrupts the Keplerian rotation, turns the energy of ordered motion into the energy of turbulence, and then into heat. A turbulent and ordered radial flow arises in a turbulent gas, which, on the one hand, carries the angular momentum of rotation outward, and on the other hand, contributes to the conversion of gravitational energy into turbulent energy. Both effects lead to significant heating of the accretion disk, which is the reason for its thermal radiation. Theoretically, the emission spectrum of the accretion disk around a supermassive BH should have maxima in the optical and ultraviolet ranges. A corona of hot material raised above the ABP can cause the appearance of X-ray photons due to the effect of backward Compton scattering. High-power radiation of HELL excites cold particles of the interstellar medium, which determines the emission lines in the spectrum. Most of the energy emitted directly by AGN can be absorbed and re-emitted in the IR (and other ranges) by dust and gas surrounding the AGN.
Periodic activation of galactic nuclei
Numerous indirect evidence is known that rotating galaxies periodically appear in an excited state, manifested in the activation of their nuclei [4] [5] [6] [7] . The past periods of activity of galaxies that are currently calm are indicated by the radial motion of the gas ejected from the nucleus, data on the metallicity of stars, indicating that star formation processes are not stationary, but periodic in nature and irregular in nature of jet-shaped emissions [8] [ 9] . The rapidly expanding ring structures observed at the center of our Galaxy at distances of 3 kpc and 2.4 kpc and the complex of molecular clouds at a distance of 300 pc from the center also support this assumption. The uneven distribution of matter within a radius of 2 pc from the center could be the result of a powerful explosion that occurred in the center of the Galaxy about 10 5 years ago [10] .
AGN Problem Status (by V. I. Pronik)
The generally accepted AGN model consists of a rotating massive central black hole and the surrounding accretion gas disk, which is a source of powerful ionizing radiation. This model qualitatively explains the observed correlation of flows in the continuous spectrum and wide hydrogen lines, as well as the existence of a delay between them. Thus, the problem of AGN is reduced to two main questions: what is the mechanism of emission of the continuous spectrum and how exactly is this radiation processed into radiation of other spectral ranges. The delay in the long-wavelength radiation of the continuum with respect to the short-wavelength observed in KrAO [11] and foreign observatories may indicate that the luminescence of most AGNs is due to strong friction and heating of the gas in the accretion disk. But there is still no reliable evidence for this. On the other hand, the luminescence of a special group of AGNs - objects of the BL Lacertae type, can be caused, according to observations made by Crimean and Finnish astronomers, exclusively by the synchrotron radiation of a relativistic gas jet directed along the axis of rotation of the disk towards the observer. The long-term spectral monitoring of AGN, carried out by some foreign observatories, as well as KrAO (since the late 1980s), together with the development of the reverberation analysis method, suggested that the emission of wide emission lines of hydrogen occurs in gas clouds moving along Keplerian orbits in approximately the same plane and forming an external drive. But there is no general agreement among experts on this subject yet. Recently, in world studies, special attention has been paid to studying the relationship between AGN radiation in the x-ray and optical ranges. Such work is being carried out in KrAO. According to Crimean astronomers, the x-ray source should be in the center above the disk, reemitting this energy in the visible region of the spectrum. The results of these and other studies are published in a book containing materials from the Astronomical Society of the Pacific Conference Series, ASPCS, vol. 360, held at the CrAO Conference on the Variability of AGN from X-ray to Radio. Despite some progress made in the study of AGNs, many problems and tasks remain unsolved, for example, such as explaining the variability of the profiles of wide hydrogen lines, the nature of their “bumpiness” in some AGNs, the kinematics and dynamics of the gas in the disk region, and increasing the accuracy of determining central masses black holes.
See also
- Galaxies
- Accretion disk
- Astrophysical hydrodynamics
Notes
- ↑ Zasov and Postnov, 2006 , p. 371.
- ↑ Zasov and Postnov, 2006 , p. 372.
- ↑ Astronomy of the 21st century -A- . Date of treatment January 9, 2014. Archived on January 9, 2014.
- ↑ Burbridge GR, Burbridge EM, Sandage AR Evidence for the occurence of violent events in the nucley of galaxies // Rev. Mod. Phys. — 1963.— 35. — P. 947-972.
- ↑ Oort JH The galactic center // Ann. Rev. Astron. Astrophys. — 1977. — 15. — P. 295-362.
- ↑ Hagen-Thorn V.A., Shevchenko I.I. Optical variability and radio structure of extragalactic sources. Evidence of recurrent activity // Astrophysics. — 1982. — 18. — P.245-254.
- ↑ Van den Bergh S. Explosions in galaxies // Vistas in Astronomy. — 1978.— 22 . — P.307-320.
- ↑ Marsakov V.A., Suchkov A.A. Metallicity function of globular clusters: evidence of three active phases in the evolution of galaxies // Letters to Astron. Zh.-1976.- 2.-S. 381-385.
- ↑ Ptuskin V.S., Khazan Y. M. Galactic center and the origin of cosmic rays // Astron. Zh. — 1981.— 58. — S.959-968.
- ↑ Gensel R. Tounes CH Physical conditions, dynamics and mass distribution in the Galaxy // Ann. Rev. Astron. Astrophys. — 1987.— 25. — P. 377-423.
- ↑ Crimean Astrophysical Observatory Archived on July 31, 2005.
Literature
- Zasov A.V., Postnov K.A. General astrophysics. - Fryazino: Century 2, 2006 .-- 496 p. - 3000 copies. - ISBN 5-85099-169-7 , UDC 52, LBC 22.6. (Retrieved February 17, 2015)
- Gorbatsky VG Introduction to the physics of galaxies and clusters of galaxies. - 1986.— M .: Nauka. - 254 p.
- Popular about active galaxies