Wednesday , July 28 2021

Black holes prefer to live in pairs

It is believed that most galaxies hide in their core a black hole with a mass of more than a million suns.

Of course, the Milky Way is no exception, and observance of star trails near the center of the galaxy allows us to calculate the mass of its central black hole, called Sagittarius A, in 4.3 million solar masses.

On the other hand, it is also noted that many large galaxies are the result of the merger of two smaller galaxies. The universe is now 13.8 billion years old, and we still see colliding galaxies in our area, but these interactions between galaxies were especially frequent during the earlier epoch of the Universe during the first billions, when space had not expanded much and the galaxies were closer. to a friend

When combined, each of the two original galaxies presents their own black holes and they slowly migrate to the center of the new galaxy that is being built. Therefore, in all these galaxies, which are the result of a merger, inside you can expect a double black hole. And astronomers accumulate clues about the presence of such pairs of monsters. For example, from the form of spectral lines created by gas emissions from the core of galaxies.

Great plasma rays

But black holes can not be seen directly, and at present, to discover these couples, researchers must resort to indirect methods. One of the most impressive manifestations of the presence of a super massive black hole is the ejection from its proximity to large plasma beams at speeds close to light velocity.

The plasma of these rays radiates mainly radio waves, and images of them, obtained by radio telescopes, are among the most fascinating in the universe.

Plasma rays can stretch for millions of light years and are straight with a single black hole. But if there is a second black hole in the neighborhood, it causes the jets to bend first or take on several morphologies, which changes the direction of rejection from which they originally originated. In some cases, these changes may consist of precession: a jet can cyclically change its emission direction with a period of the order of hundreds of thousands of years.


Martin Krause from the universities of Hertfordshire (United Kingdom) and Tasmania (Australia), together with an international group of astronomers, studied the geometry of jets outputting a group of active galaxies, choosing those that are not quite simple, indicating the presence of double black holes. So far, they have observed 33 of these galaxies, and rays of 24 of them (73%) showed some kind of curvature or changes in linear pathways. For example, in the case of the Swan-A galaxy, the beam differs very strongly from its axis, which can be explained by the theoretical model of a pair of black holes rotating around each other for a period of 18 years.

These results, along with some other previously obtained other groups, confirm that there is a very important population of double black holes in the largest galaxies. However, some astronomers point out that changes in the direction of the beam are not a definite sign of a binary hole. In particular, any slopes or irregularities in the accretion disk that would exist around a super-massive black hole (the disc from which ultimately the energy of the jet continues) may also change the straight path for such plasma excretions.

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