American and Australian astrophysicists have discovered a candidate for medium-mass black holes. They got this name because they are heavier than ordinary - that is, those formed as a result of the gravitational collapse of stars - objects, but lighter than supermassive black holes, usually located in the active nuclei of large galaxies. The origin of the unusual objects is still unclear. "Lenta.ru" talks about black holes of intermediate masses and the discovery of scientists.
Most of the black holes known to scientists - that is, objects that no matter can leave (ignoring quantum effects) - are either stellar mass black holes or supermassive black holes. The origin of these gravitational objects is roughly clear to astronomers. The first, as their name implies, represent the final stage in the evolution of heavy luminaries, when thermonuclear reactions cease in their depths. They are so heavy that they do not turn into white dwarfs or neutron stars.
Small stars like the Sun turn into white dwarfs. Their force of gravitational compression is balanced by the electromagnetic repulsion of the electron-nuclear plasma. In heavier stars, gravity is constrained by the pressure of nuclear matter, resulting in neutron stars. The core of such objects is formed by a neutron liquid, which is covered with a thin plasma layer of electrons and heavy nuclei. Finally, the heaviest luminaries turn into black holes, which is perfectly described by general relativity and statistical physics.
Globular star cluster 47 Toucan
Photo: NASA / ESA / Hubble Heritage
The limiting value of the mass of the white dwarf, which prevents it from turning into a neutron star, was estimated in 1932 by the Indian astrophysicist Subramanian Chandrasekhar. This parameter is calculated from the equilibrium condition of the degenerate electron gas and gravitational forces. The current value of the Chandrasekhar limit is estimated at about 1.4 solar masses. The upper limit on the mass of a neutron star, at which it does not turn into a black hole, is called the Oppenheimer-Volkov limit. It is determined from the equilibrium condition of the pressure of the degenerate neutron gas and the forces of gravity. In 1939, scientists received its value at 0.7 solar masses; modern estimates range from 1.5 to 3.0.
The most massive stars are 200-300 times heavier than the Sun. As a rule, the mass of a black hole originating from a star does not exceed this order of magnitude. At the other end of the scale are supermassive black holes - they are hundreds of thousands or even tens of billions of times heavier than the Sun. Usually such monsters are located in the active centers of large galaxies and have a decisive influence on them. Despite the fact that the origin of supermassive black holes also raises many questions, to date, enough such objects (more strictly - candidates for them) have been discovered so as not to doubt their existence.
Promotional video:
For example, in the center of the Milky Way, at a distance of 7.86 kiloparsecs from Earth, is the heaviest object in the Galaxy - the supermassive black hole Sagittarius A *, which is more than four million times heavier than the Sun. In the nearby large star system, the Andromeda Nebula, is an even heavier object: a supermassive black hole, which is probably 140 million times heavier than the Sun. Astronomers estimate that in about four billion years, a supermassive black hole from the Andromeda Nebula will swallow one from the Milky Way.
Medium mass black hole (artist imagined)
Image: CfA / M. Weiss
This mechanism points to the most likely way giant black holes form - they simply absorb all the matter around them. However, the question remains: do there exist in nature black holes of intermediate masses - between stellar and superheavy? Observations of recent years, including those published in the recent issue of the journal Nature, confirm this. In the publication, the authors reported the discovery of a probable candidate for medium-mass black holes in the center of globular star cluster 47 Toucan (NGC 104). Estimates show that it is about 2.2 thousand times heavier than the Sun.
Cluster 47 Toucan is located 13 thousand light-years from Earth in the constellation Toucan. This set of gravitationally bound luminaries is distinguished by its great age (12 billion years) and extremely high brightness among such objects (second only to the omega Centauri). NGC 104 contains thousands of stars, confined to a conditional sphere 120 light-years in diameter (three orders of magnitude smaller than the diameter of the Milky Way's disk). Also in 47 Toucan, there are about twenty pulsars - they became the main object of research by scientists.
Previous searches in the center of NGC 104 for a black hole were unsuccessful. Such objects reveal themselves in an indirect way, by the characteristic X-rays emanating from the accretion disk around them, formed by the heated gas. Meanwhile, the center of NGC 104 contains almost no gas. On the other hand, a black hole can be detected by its effect on the stars rotating in its vicinity - something like this is possible to study Sagittarius A *. However, even here, scientists were faced with a problem - the center of NGC 104 contains too many stars to be able to understand their individual movements.
Parks radio telescope
Photo: David McClenaghan / CSIRO
Scientists have tried to get around both difficulties, while at the same time not abandoning the usual methods of detecting black holes. First, astronomers analyzed the dynamics of the stars of the entire globular cluster as a whole, and not just those stars that are close to its center. To do this, the authors took data on the dynamics of the luminaries of 47 Toucan, collected during observations by the Australian Parkes radio observatory. The scientists used the obtained information for computer modeling in the framework of the gravitational problem of N bodies. It showed that there is something at the center of NGC 104 that resembles a medium-mass black hole in characteristics. However, this was not enough.
The researchers decided to test their findings on pulsars - compact remnants of dead stars, the radio signals of which astronomers have learned to track quite well. If NGC 104 contains a medium-mass black hole, then pulsars cannot be located too close to the center of 47 Toucan - and vice versa. As expected by the authors, the first scenario was confirmed: the location of the pulsars in NGC 104 correlates well with the fact that there is a black hole of average mass in the center of the cluster.
The authors believe that gravitational objects of this kind can be located in the centers of other globular clusters - probably, where they are already or are not yet sought. This will require careful consideration of each of these clusters. What role do intermediate-mass black holes play and how did they arise? It is not yet known for certain. Despite the many options for their further evolution, study co-author Bulent Kiziltan believes that "they may be the original seeds that grew into the monsters that we see today in the centers of galaxies."
Yuri Sukhov
