How many stars destroyed by the merger of two galaxies

Scientists from MIPT, Oxford University and Physical Institute. P. N. Statistics and data analysis estimated how many stars destroyed by the merger of two galaxies, the centers of which are supermassive black holes. Astrophysicists have checked whether the gravitational effects of two approaching black holes to explain why the observed value of the capture rate of the stars is less than that obtained from a simple theoretical model.

In an article published in The Astrophysical Journal, researchers evaluated the mutual influence of dynamic mechanisms on the rate of capture of stars. Spoiler: advanced theoretical model gave an even larger discrepancy with the observations, and the authors put forward the hypothesis that the destruction of stars in the centers of galaxies might happen unbeknownst to us.

Grab the stars

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)

The events of the tidal destruction of stars, or TDE (eng. tidal disruption event), the only currently available way to obtain some information from the inactive centers of galaxies. 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 very high temperatures and which can be “seen” with telescopes. 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%) is “silent”, because there is no gas accumulations. The only source of matter for black holes are 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). At the moment were only about fifty flares from TDE, but on average one galaxy, one star collapses once in 10-100 thousand years. Based on these data, scientists are trying to build a credible model of what happens in the inactive centers of galaxies.

The simulated disruption of a star:

A spherical galaxy in a vacuum

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 is observed. This model is updated to take account of different factors, such as inequality of the masses of stars, but they only increase the capture rate.

Star speed which was in the “loss cone”
BH — black hole, gsfv — radius capture

The effect of a slingshot

At this point 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. This can be presented on 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, thus starting 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.

Merging black holes
Inspiral — the initial stage, during which black holes rotate around a common center of mass and gradually approach each other. Merger — pre-merger, during which the emitted gravitational waves (below shows their registration). Ringdown — in the final stage, a single black hole

Aspherical galaxy vacuum

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 has done the calculations and looked at how changing the rate of capture for different masses of the Central black hole, for different geometries of the Central clusters, under different initial conditions.

The collision of two galaxies, our milky Way and Andromeda (computer simulation):

More destruction!

Disruption of a star by tidal forces near a black hole

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 with another — raises the question of why the observed rate of events tidal destruction are lower than predicted by theoretical models.

Kirill Lezhnin, author, researcher, laboratory of astrophysics and physics of nonlinear processes of MIPT, commented: “As we have shown, 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.”

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