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A European instrument, Gravity, has detected orbiting matter in the immediate vicinity of Sagittarius A *, thus providing further proof of the existence of this ogre of four million solar mbades nestled in the heart of our Milky Way.
Like many galaxies in the universe, our Milky Way is certainly home to a supermbadive central black hole in its center. The latter would concentrate 4 million times the mbad of our Sun in a sphere at most 20 times bigger. Only problem: we have never been able to observe it. To be more precise, we have never actually observed directly any black holes so far, whether supermbadive or not, in our galaxy or in another. Only indirect indices have gradually convinced us of their existence. The detection in 2016 of gravitational waves emitted by the fusion of two of these objects will have convinced the most skeptical, if any, of their reality.
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Astronomers, however, would like to know what these objects look like. Or at least their environment. Since nothing escapes their gravitational hold below a certain distance, not even light, they delimit indeed a sphere of absolute blackness. What is not very interesting in terms of imagery … But there is very likely around the black hole a ring of gas and dust that orbits at a very high speed by emitting light. Astrophysicists have good reason to think that it is the heating of this material during its collapse towards the black hole which emits the intense radiations betraying their presence.
Several projects are currently underway to try to find what is hidden in the immediate vicinity of the central black hole of our galaxy, Sagittarius A *, the closest and easiest to study. Gravity, a new generation instrument installed in Chile on the Very Large Telescope (VLT) of the European Southern Observatory (ESO), for example, has just unveiled exceptional results. This device, which combines the light of the four telescopes of the VLT, is so precise that it would be able to detect a displacement of the thickness of a hair at 1000 km distance. The latter was able to detect the movement of three bursts of light that occurred between May and July in the near infrared.
The modeling below represents a dive into the galactic center. We first distinguish the round of stars around the central black hole, then the trajectory of the nearest star that has never touched it, and finally the movement of these luminous bursts, represented by a red cloud:
"They describe an arc of a circle at a considerable speed, equivalent to 30% of the speed of light (about 100,000 km / s, or 360 million km / h)", decrypts Thibaut Paumard, astrophysicist at LESIA (CNRS / Observatoire de Paris), one of the laboratories involved in the Gravity collaboration. "Whatever the origin of these emissions, they were only a few rays away from the black hole." This is several hundred times closer than the closest object ever seen in its vicinity (see below). below).
READ ALSO – Are thousands of black holes hiding in the center of our galaxy?
The origin of the infra-red bursts revealed today is not yet perfectly clear. "We are in front of a hot spot whose temperature could reach several billion degrees, if not more," said Guy Perrin, astronomer at the Paris Observatory and co-investigator of the Gravity instrument. "We suspect magnetic phenomena to be at the origin of this very abrupt and very localized heating."
"It has always been one of our dreams (to detect this type of hot spot very close to the central black hole, Ed), but we did not dare to imagine that it could be realized as quickly (less than three years after the first one). light recorded by Gravity, Ed), "says Reinhard Genzel, director Max Planck Institute for Extraterrestrial Physics, the laboratory responsible for the instrument. "This result is a resounding confirmation of the supermbadive black hole paradigm."
A "radio" portrait of Sagittarius A * in the coming years?
These observations make it possible to confirm that a mbad of 4 million solar mbades located in the center of the Milky Way (these are the trajectories of the central stars that allowed to reach this figure before) are located in a sphere of less than 60 million kilometers of radius. A constraint that now excludes almost all the badumptions, usually very exotic, that remained. "Only a boson star (a family of particles to which photons, gluons or the famous Higgs boson, Ed) belong, which could still explain such a concentration of mbad, but these very hypothetical objects still require the discovery of the said boson, which would be much more mbadive than all those we know today, "says Guy Perrin.
Researchers know that 10 or 100 times more intense bursts can occur because this has happened in the past. These could be linked to "hot spots" orbiting even closer to the black hole. To obtain a true "portrait" of the central black hole of the galaxy, it is yet another technique that must be implemented. Researchers from around the world who have been part of the Event Horizon Telescope (EHT) collaboration have been steadily targeting all the world's largest radio telescopes since 2017 towards the center of the Milky Way. They hope to produce an image of "the shadow of the black hole" that emerges in the middle of the matter that ends to collapse by emitting radio waves.
"Gravity's outstanding results seem to indicate that the axis of rotation of the black hole is almost facing us, which would be a remarkable coincidence," says Heino Falcke, professor of radio astronomy and astroparticle physics at Radboud University. of Nijmegen (The Netherlands), and one of the founders of the EHT project. "It's also the most favorable situation to realize the portrait that we are trying to produce."
A star flashed at 25 million km / h last May
Artist's impression of the star S2 and its reddening as it pbades close to Sagittarius A * (this representation is not to scale). European Southern Observatory / ESO / M. Kornmesser
The Gravity instrument installed on the VLT had already unveiled some very interesting results this summer. The researchers of the collaboration indeed measured very precisely the trajectory of the star S2, the closest to the central black hole of our galaxy never identified. The latter takes 16 years to make the round on a very elliptical trajectory and pbaded closer in 2018 precisely. At that time, it was more than 25 million km / h, nearly 3% of the speed of light.
On this occasion, the researchers had noticed a very marked "gravitational blush", the light of the star being in a way "stretched" by the power of the gravitational field of the black hole. This observation further confirmed the validity of Einstein's theory of relativity in a largely unexplored regime.
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