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How many stars the “eat” black holes

Black holes of the merging galaxies absorb smaller stars than I should.

In the center of most galaxies is at least one supermassive black hole surrounded by a dense Central star cluster – the core of the galaxy. The black hole then the black that it does not radiate. But emits a substance during the fall in the supermassive object is heated to extremely high temperatures this radiation can be “seen” with telescopes.


Disruption of a star by tidal forces near a black hole. (Source: NASA/CXC/M. Weiss; X-ray: NASA / CXC / MPE / S. Komossa et al.; Optical: ESO / MPE / S. Komossa)

Disruption of a star by tidal forces. The matter of the star falls into the black hole and emits x-rays. Below are data from three telescopes (Y — brightness x-rays, the X — axis is wavelength). (Source: NASA.)

Star speed which was in the “loss cone”. BH — black hole; gsfv — radius of capture. (Source: website article VIA for calculations http://td.lpi .ru/~eugvas/losscone/)

The merger of black holes. Inspiral – start, black holes gradually moving closer to each other. Merger – the merger with the emission of gravitational waves (below shows their registration). Ringdown – in the final single black hole. (Source: LIGO, NSF, Aurore Simonnet”

In active galaxies there is gas accumulations that feed the black hole and thereby allow her to notice. However, most of the galaxies (about 90%) are inactive – they are “silent” because there are no gas accumulations. In inactive galaxies, the only source of matter for black holes – stars that periodically get too close to her. When such “careless” star is destroyed by tidal forces, the astronomers record the event of tidal destruction (TDE) (eng. tidal disruption event; the simulated disruption of a star can be viewed here).

At the moment were only about fifty flares from TDE, but on average one galaxy, one star collapses once in 10-100 thousand years. And if we where can I get information about what is happening in the center of an inactive galaxy, only from TDE.

Using the data received from TDE, astrophysicists are building a model of the galaxy, and the simplest theoretical model is a galaxy with a spherical nucleus in the center of which is a supermassive black hole. Around her revolve the stars that diffuse in space velocity, in other words, change its direction when passing near each other, just as changing its trajectory billiard balls in a collision.

But if a billiard ball to fall into the pocket, must move directly toward it, the star over a wide range of areas: its velocity vector must be within the so-called the loss-cone, then it will be captured and destroyed by the gravity of a black hole. In accordance with this simple model, the capture of stars should occur approximately every 1-10 thousand years, that is more often than it actually happens. The model can be supplemented, taking into account, for example, such as differences in stellar masses, but the augmented model the rate of capture is increasing.

So far in the scientific literature has only discussed one mechanism that can reduce the rate of capture of stars – if the majority of stars with small angular momentum will disappear. What this means can be represented by the example of gas diffusion: let the chaotic motion of gas molecules absorbed on the wall of the vessel, then, if you remove next to the wall, the rows of molecules, the number of absorbed molecules per unit time will decrease as the far more molecules need to reach the wall. As with a black hole: if you remove the stars from the center, the capture rate will decrease. The stars, of course, you cannot just take and clean up, but if the center of the galaxy is dual black hole, that is incident on it, the star can be ejected out of the galaxy, making the so-called gravity assist. Another name for this phenomenon – the effect of a slingshot.

According to the law of conservation of energy, when the star is accelerated, that is, an addition to its kinetic energy, the energy of binary black hole must decrease. As a result, black holes become closer to each other and begin to merge. And, as confirmed by the recent sensational discovery in the last stages of merging black holes part of the energy is radiated in form of gravitational waves. (Here you can see a computer simulation of collision of galaxies the milky Way and Andromeda.)

Although merging galaxies, the capture rate may decrease, at the same time observed the opposite effect. – Any of the galaxy, which originated in the merger, is a little different from sphere. In non-spherical the core of a star is stronger than shuffled, so the more star orbits is near a black hole. Because of the merger, the capture rate will increase, although the effect of the slingshot need to reduce it. To understand how these competing effects will affect the capture rate, MIPT graduates Kirill Lezhnin and Evgeny Vasiliev analyzed the changes in the rate of capture for different masses of the Central black hole, for different geometries of the Central clusters, under different initial conditions.

It turned out that the removal of stars from the center of gravity maneuver is only relevant for “spherical galaxy in a vacuum”, but with the merger of the galaxies will take place at least a small sphericity. In the end, the destruction should be an average of one star for 10 thousand years.

On the one hand, this is consistent with previous results of theoretical calculations, and on the other – raises the question of why the observed rate of events tidal destruction are lower than predicted by theoretical models. The results obtained by the researchers and their colleagues from Oxford University and Physical Institute. P. N. Lebedev Sciences published in The Astrophysical Journal.

According to Kirill Lezhnin, they were able to show that “…the observed low capture rate cannot be explained by the effect of a slingshot. So, we need to find a mechanism beyond the study of stellar dynamics. Perhaps the events of the tidal destruction happens as much as we thought. Then you have to think why we can not see.”

According to the press service of MIPT.

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