Quasar

In addition to the modern definition, there was also the original one ^[18] : “Quasar (quasi-star object) is a class of celestial objects that, in the optical range, resemble a star, but have strong radio emission and extremely small angular dimensions (less than 10 ″)”; star-like self-emitting cosmic body, in mass and luminosity many times larger than the Sun ^{[19] [20]} .

The initial definition was formed in the late 1950s - early 1960s, when the first quasars were discovered and their study had just begun. This definition is generally true, but over time, radio-quiet quasars have been discovered that do not produce strong radio emission ^{[18] [21]} ; As of 2004, these are about 90% of the known quasars.

The history of quasars began with the Jodrell Bank radio observatory conducted by the program of measuring the apparent angular dimensions of radio sources.

The first quasar, 3C 48 , was discovered in the late 1950s by Allan Sandage and Thomas Matthews during a sky radio survey. In 1963, 5 quasars were already known. A new type of objects combined some anomalous properties, which at that time could not be explained. They emitted a large amount of broad-spectrum radiation, but most of them were not optically detected, although in some cases it was possible to identify a weak and point object, similar to a distant star. The spectral lines that identify the chemical elements that make up the object were also extremely strange and could not be decomposed into the spectra of all the elements known at the time and their various ionized states.

In the same year, the Dutch astronomer Martin Schmidt proved that the lines in the spectra of quasars were strongly shifted to the red . The strange 3C 48 spectrum was quickly identified by Schmidt, Greenstein and Oka as lines of hydrogen and magnesium strongly shifted to the red part of the spectrum. If this were due to the physical movement of the "star", then 3C 273 moved away from us at a tremendous speed, about 47,000 km / s, far exceeding the speed of any known star ^[22] . Also, the extreme speed would not help explain the huge radio emissions of 3C 273. If the redshift was cosmological (now known to be correct), a large distance meant that 3C 273 was much brighter than any galaxy, but much more compact. Almost immediately, on April 9, 1963, Yu. N. Efremov and A. S. Sharov from the photometric measurements of the 3C 273 source images revealed the brightness variability of quasars with a period of only a few days ^{[23] [24]} . Irregular brightness of quasars on time scales of less than a day indicates that the region of generation of their radiation has a small size, comparable to the size of the solar system , but their brightness is many times greater than the brightness of ordinary galaxies. In addition, the 3C 273 was bright enough to be found in archival photographs of the 1900s; it was discovered that it varies at an annual time scale, implying that much of the light was emitted from a region less than 1 light year in size, tiny compared to the galaxy. Assuming that this redshift is caused by the effect of a cosmological redshift resulting from the removal of quasars, the distance to them was determined by the Hubble law . One of the closest and brightest quasar 3C 273 has a brightness of about 13 ^{m [25]} and a redshift z = 0.158 ^[26] (which corresponds to a distance of about 3 billion light years ) ^[27] . The most distant quasars, due to their gigantic luminosity, which is hundreds of times greater than the luminosity of ordinary galaxies, are recorded with the help of radio telescopes at a distance of more than 12 billion St. years As of July 2011, the most distant quasar ( ULAS J112001.48 + 064124.3 ) was at a distance of about 13 billion sv. years from the earth ^[28] .

It is very difficult to determine the exact number of quasars discovered to date. This is explained, on the one hand, by the constant discovery of new quasars, and on the other, by the lack of a clear boundary between quasars and other types of active galaxies . In the Hewitt-Burbridge list published in 1987, the number of quasars is 3594. In 2005, a group of astronomers used data from 195,000 quasars in their study ^[29] .

Quasars immediately from the moment of their discovery caused a lot of discussions and disputes in the scientific community. The small dimensions were confirmed by interferometry and the observation of the speed with which the quasar as a whole varied in power, and the inability to see even the most powerful optical telescopes anything more than weak stellar point sources. But if the objects were small in size and located far in space, their energy release would be extremely huge and difficult to explain. On the contrary, if, with their size, they were much closer to our galaxy, then it would be easy to explain their apparent power, but then it is difficult to explain their redshifts and the absence of detectable motions against the background of the Universe (parallax).

If the measured redshift was due to expansion, this would support the interpretation of very distant objects with unusually high brightness and output power far exceeding any object seen to date. This extreme brightness also explains the large radio signal. Schmidt came to the conclusion that 3C 273 can be either a separate star with a diameter of about 10 km inside (or near) our galaxy, or a distant active galactic core. He stated that the assumption of a distant and extremely powerful object would most likely be correct ^[22] .

The explanation of the strong redshift was not generally accepted at that time. The main problem was the enormous amount of energy that these objects would have to radiate if they were at such a distance. In the 1960s, no generally accepted known mechanism could explain this. The currently accepted explanation that this is due to a substance falling in an accretion disk into a supermassive black hole was proposed only in 1964 by Zeldovich and Edwin Salpeter ^[30] , and even then it was rejected by many astronomers, because in 1960 In the early 1990s, the existence of black holes was still widely regarded as theoretical and too exotic, and it has not yet been confirmed that many galaxies (including ours) have supermassive black holes in their center. Strange spectral lines in their radiation and the rate of change observed in some quasars, many astronomers and cosmologists explained that the objects were relatively small and, therefore, possibly bright, massive, but not so far away; accordingly, their redshifts were not due to the distance or speed of moving away from us due to the expansion of the Universe, but due to some other reason or an unknown process, which means that quasars were not really such bright objects at extreme distances.

Various explanations were proposed in the 1960s and 1970s, and each had its drawbacks. It was suggested that quasars are nearby objects, and that their redshift is not connected with the expansion of space (explained by the special theory of relativity ), but with light coming from the deep gravitational pit (gravitational redshift is explained by the general theory of relativity ). This would require a massive object, which would also explain the high brightness. However, a star with sufficient mass to obtain a measured redshift will be unstable and will exceed the Hayashi limit ^[31] . Quasars also show forbidden spectral emission lines that were previously visible only in low-density hot gas nebulae that would be too diffuse to simultaneously generate the observed power and fit into a deep gravity well ^[32] . There were also serious cosmological concerns about the idea of ​​distant quasars. One strong argument against them was that they implied energies that far exceeded the known energy conversion processes, including nuclear fusion. There were some assumptions that quasars were made from some previously unknown form of stable areas of antimatter and we observe the area of ​​its annihilation with ordinary matter, and this could explain their brightness ^[33] . Others have suggested that quasars were the end of a white hole of a wormhole ^{[34] [35]} or a chain reaction of numerous supernovae.

Eventually, starting around the 1970s, many of the evidence (including the first X-ray space observatories, knowledge of black holes, and modern cosmology models) gradually demonstrated that the quasar redshifts are genuine, and because of the expansion of space that quasars in fact as powerful and as distant as Schmidt and some other astronomers have suggested, and that their source of energy is matter from an accretion disk falling on a supermassive black hole. This assumption was reinforced by the most important data of optical and X-ray observation of quasar host galaxies, the discovery of "intermediate" absorption lines explaining various spectral anomalies, observations of gravitational lensing, and the discovery by Peterson and Gann in 1971 of the fact that galaxies containing quasars showed the same red the shift that quasars did and the discovery by Christian in 1973 that the “foggy” environment of many quasars corresponded to a less luminous host galaxy.

This model also fits well with other observations, which suggest that many or even most of the galaxies have a massive central black hole. It also explains why quasars are more common in the early universe: when a quasar absorbs matter from its accretion disk, a moment comes when there is little matter in the vicinity, and the flow of energy falls or stops, and then the quasar becomes an ordinary galaxy.

The mechanism of energy production in the accretion disk was finally modeled in the 1970s, and evidence of the existence of black holes themselves was also updated with new data (including evidence that supermassive black holes can be found in the centers of our own and many other galaxies), which allowed solve the problem of quasars.

Quasars are located in the center of active galaxies and are among the brightest objects known in the Universe, radiating a thousand times more energy than the Milky Way, which contains 200–400 billion stars. The bolometric (integral over the whole spectrum ) quasar luminosity can reach 10 ⁴⁶ —10 ⁴⁷ erg / s ^[36] . On average, a quasar produces about 10 trillion times more energy per second than our Sun (and a million times more energy than the most powerful known star), and it has radiation variability in all wavelength ranges ^[18] . This radiation is radiated over the entire electromagnetic spectrum, almost evenly, from X-rays to the far infrared range with a peak in the ultraviolet-optical ranges, with some quasars also being strong sources of radio emission and gamma radiation. Using high-resolution images obtained from ground-based telescopes and the Hubble Space Telescope, in some cases, host galaxies surrounding quasars ^[37] were discovered. These galaxies are usually too dull to be seen in the bright light of a quasar. Most of the quasars, with the exception of 3C 273 , the average apparent magnitude of which is 12.9, cannot be seen with small telescopes.

The mechanism of quasar radiation is the accretion of matter in supermassive black holes in the nuclei of distant galaxies. Light and other radiation cannot escape from the inside of the black hole event horizon, but the energy created by the quasar is generated outside when gravity and tremendous friction (due to the viscosity of the gas in the accretion disk) heats the substance to very high temperatures. With this mechanism, from 6% to 32% of the mass of the object can be converted into radiation energy, which is an order of magnitude higher compared to 0.7% for the process of thermonuclear fusion in the proton-proton cycle , which prevails in stars similar to the Sun. The central masses of quasars were measured in quasars using reverberation mapping and are in the range from 10 ⁵ to 10 ⁹ solar masses. It has been confirmed that several dozen nearby large galaxies, including our own Milky Way galaxy, which do not have an active center and do not show any activity similar to quasars, contain in their cores a similar supermassive black hole (the center of the galaxy). Thus, it is now believed that although all large galaxies have a black hole of this type, only a small part has enough material in its vicinity to become active and radiate energy in such a way that it can be viewed as a quasar ^[38] .

It also explains why quasars were more common in the early Universe, since this release of energy ends when a supermassive black hole absorbs all the gas and dust around it. This means that it is possible that most galaxies, including the Milky Way, have passed an active stage, looking like a quasar or some other class of active galaxy, which depended on the black hole's mass and accretion rate, and are now at rest, because they lack the substance in the immediate vicinity to generate radiation. For our galaxy, there is evidence of black hole activity in the past, such as Fermi bubbles.

A substance accumulating around a black hole is unlikely to fall directly into it, but due to some initial angular momentum, the substance will accumulate in the accretion disk, and due to the law of conservation of angular momentum, the closer it is to the black hole, the rotational speeds will be higher, actually approaching to the speed of light. Quasars can also re-ignite when ordinary galaxies merge and the black hole fills with a fresh source of matter. It has been suggested that a quasar may form when the Andromeda galaxy collides with our own Milky Way galaxy in about 3–5 billion years ^{[39] [40] [41]} .

Glitter Variations

Many quasars change their luminosity in short periods of time. This is, apparently, one of the fundamental properties of quasars (the shortest variation with a period t ≈ 1 h, the maximum brightness variations 50 times). Since the size of an object that is variable in terms of brightness cannot exceed ct ( c is the speed of light) , the dimensions of the quasars (or their active parts) are very small - of the order of the light hours.

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