2023-08-07 17:36:50
The Hubble Telescope has shown us that there are many instances of large colliding galaxies in the observable Universe, and have been for billions of years.
However, quickly, following the discovery of quasars regarding 60 years ago, people began to think that large black holes might be hiding in the hearts of galaxies. Thus, as early as 1964, the great astrophysicists Zel’dovich, Novikov and Salpeter had proposed that quasars, more generally the active nuclei of galaxies, were supermassive black holes accreting matter. In 1971, Donald Lynden-Bell et Martin Rees proposed for their part that there was one in the heart of the Milky Way and more generally in many others. At least since the early 1990s, it seems clear that most large galaxies have one of these compact stars at their center.
This numerical simulation brings astrophysicists one step closer to understanding the kinds of light signals produced when two supermassive black holes surrounded by matter, which are millions to billions of times the mass of the Sun, spiral toward collision. This simulation fully incorporates the physical effects of Einstein’s theory of general relativity and it shows that the gas in such systems will glow primarily in ultraviolet light and X-rays. For a fairly accurate French translation, click on the white rectangle at bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © Nasa’s Goddard Space Flight Center
Binary black holes produced by galaxy mergers
From then on, one might imagine that during collisions followed by fusion between two large galaxies, the formation of a supermassive binary black hole might follow, the two original black holes present in the two initial galaxies being driven by certain processes to plunge towards the heart of the new galaxy. As a bonus, these two black holes were also to start producing copious gravitational waves and eventually, due to the energy dissipated by these waves, by colliding.
It is suspected that it is precisely these waves that have been detected for several years by members of theInternational Pulsar Timing Array (ipta). One might therefore quite naturally ask the question of the existence of a supermassive binary black hole in the center of the Milky Way.
Some answers have just been provided on this subject. via an article on arXiv by a team of researchers including the Nobel Prize in Physics Andrea Ghez, who helped to establish the existence of the black hole behind the Sagittarius A* radiosource at the heart of our Galaxy, the American physicist Clifford Will, who is one of the great masters of the various observational tests of general relativity, and Aurélien Hees, stationed at the Paris Observatory and who also works on tests of the theories of gravitation extending Einstein’s theory and in particular with the black hole center of the Milky Way.
Andrea Ghez, professor of physics and astronomy at Ucla, California, tells us regarding her work on the central supermassive black hole of the Milky Way, work that won her the Nobel Prize in Physics. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. ©
Let’s clarify things a bit. A supermassive black hole contains at least a million solar masses, and it is believed that stellar black holes of a few tens of stellar masses at most can form by gravitational collapse of massive stars. We can climb en masse by merging stellar black holes, but we know that there are also so-called black holes with masses intermediate between those of stellar black holes and those of supermassive black holes.
We also know from observations a few cases of binary supermassive black holes in galaxies.
Orbiting Stars Betray Giant Black Holes
It should also be remembered that Reinhard Genzel and Andrea Ghez received the 2020 Nobel Prize in Physics for studying for almost two decades the movements of stars in the heart of the Milky Way around the region containing the radiosource. Sagittarius A*.
The particularly rapid movements of these stars, such as the one called S0-2, or S2, showed that there was a mass of just over 4 million solar masses, hardly radiating and in a volume so small that the he most rational explanation was indeed that it was a black hole described in first approximation by the theory of general relativity.
It finally turned out that the movements of these stars might serve as a remarkable laboratory for testing new physics, such as the existence of a fifth force for example.
A video on the observations of stars around the supermassive black hole of the Milky Way with the VLT. Real images are mixed with synthetic images. © ESO
In the end, the presence of a second supermassive black hole in orbit around Sagittarius A* would have already been highlighted for several years already. The article published today, which also uses the relativistic motions of stars around the black hole which was even imaged by the members of the collaboration Event Horizon Telescopetherefore in fact relates to the presence in orbit around Sagittarius A* of a black hole of intermediate mass.
To be precise, astrophysicists have come to the conclusion that between 1,000 AU (regarding the orbital radius of S2, the closest known star to the supermassive black hole) and 4,000 AU away from Sagittarius A*, there is no there may be intermediate black holes with masses between 1,000 and 100,000 solar masses.
If an intermediate-mass black hole exists and is closer to Sagittarius A* than S2, then its mass cannot exceed 400 times that of the Sun.
What is the history of gravity? What is the quantum theory of gravity? Why search for alternative theories of gravity? What tools can be developed to evaluate these new theories? During a presentation at the ENS, Aurélien Hees reviews the various tests of general relativity carried out previously and gives an overview of those which will be carried out in the future. © Ecole Normale Supérieure – PSL
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