Here is the first image of the giant black hole in the center of the Milky Way!

4 p.m. And here is a comparison of the two black holes from which we were able to obtain images, M87* and Sagittarius A*!  » We have two completely different types of galaxies and two masses very different black holes, but near the edge of these black holes they look surprisingly similar “says Sera Markoff in the communicated of the CNRS, co-president of the scientific council of the EHT and professor ofastrophysics theory at the University of Amsterdam, the Netherlands.  » This tells us that the general relativity governs these objects up close, and that any difference we see further away must be due to dissimilarities in the matter surrounding the black holes. »

3:45 p.m. Many animations and information are added. In particular, multiple images that explain how important the work of the researchers was: they averaged a gargantuan quantity of images. Now, they intend to compare the two images obtained, which correspond to very different black holes: one weighs 4 million solar masses – Sagittarius A*, while the other represents 6.5 billion solar masses, M87*! « We have images for two black holes – one at the large end and one at the small end of supermassive black holes in theUniverse – we can therefore go much further in testing the behavior of the gravity in these extreme environments than ever before. said Keiichi Asada, co-author of one of the six studies published in the journal. The Astrophysical Journal Lettersand researcher at the Institute of Astronomy and Astrophysics in Taipei.

UArizona astronomers have helped capture the first image of a supermassive black hole at the center of our Milky Way galaxy as part of the international @ehtelescope. Known as Sagittarius A*, this black hole is 4 million times more massive than the sun. https://t.co/pAd8WNfo6Qpic.twitter.com/t8gw7SgXTX

— University of Arizona (@uarizona) May 12, 2022

3:35 p.m. We suspected it but the researchers confirm it: the Sagittarius A* source is indeed a black hole ! « We were amazed at how well the size of the ring matched the predictions of the theory of general relativity ofEinstein »said Geoffrey Bower, EHT project scientist, Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. The accretion disk orbits Sagittarius A* in just a few minutes, compared to several days or even weeks for M87*, the scientists explain. This means that the brightness changed rapidly during the observations made, then complicating the calculations to obtain an image.

It is 3:07 p.m.: this is a world first, the EHT researchers (Event Horizon Telescope) and theESO (European Southern Observatory) have just unveiled the very first image of the Sagittarius A* black hole! “We have been so close, many times, before. » said in the press conference ESO President Xavier Barcons. Located in the center of our Galaxy Milky Way at 27,000 light years from us, this astronomical monster of 4 million solar masses has a diameter of its horizon that reaches 6 million kilometers, or about 15 times the Earth-Moon distance.

Synchronized telescopes all over the world and 5 years of calculations!

Obtaining this image was not easy! It all started with an observation campaign in April 2017 by interferometry very long base, also called VLBI. This observation technique consists of the simultaneous use of many radio telescopes in the world, in order to create the equivalent of a gigantic interferometer the size of the Earth. By doing so, the resolution angular obtained which defines the smallest angular size that researchers are able to observe in detail, becomes so small that many normally invisible objects become visible. This is the case with M87* and Sagittarius A*, two black holes whose apparent diameter is similar, and which require a resolution that the VLBI can reach. In effect, M87* is both much more massive and much further away than Sgr A*, with its 6.5 billion solar masses and its distance of 50 million light-years, so its diameter visible from Earth is equivalent to that of Sgr A* .

The EHT collaboration makes it possible, thanks to this method, to achieve the greatest angular resolution power in the world. Inside were there during the observation in 2017 eight radio telescopes located all over the globe: Mexico, Chile, Antarctic, United States, Spain. In each case, the antenna radio is located at high altitude, in order to eliminate air pollution as much as possible. This process also requires a lot of preparation, because the synchronization must be perfect. Thus, the only observation campaign by this method took place in 2017, more precisely from April 4 to 14, 2017. Then comes the data analysis! This phase is just as difficult as the first, and requires super computers, called correlators. In total, more than 350 people took part in this technological feat. Thus, in 2019, the very first image of M87* has been unveiled, but not that of Sagittarius A*, which the scientific community was also expecting.

Ultimately, the researchers want to test general relativity

But what does this image bring, scientifically speaking? Many things. First, getting the true appearance of a black hole allows it to be compared with models and simulations existing, in order to correct them. It therefore allows to know more about the black hole physics. Indeed, obtaining the dimension of the horizon, also called the shadow of the black hole, and the luminosity which surrounds it gives many clues to its properties: its size of course, its mass, but also and above all the dynamics of its disc. of accretion. This is made up of the matter that the black hole attracts towards it, heated to a very high temperature and which spins at gears relativists. Indeed, it only takes 4 minutes and 30 seconds for the dust particles that make up the disks to orbit the black hole, which, let’s remember, has a horizon diameter of about 6 million kilometers!

It also allows the astronomers to test the general relativity established by Einstein in 1916, in the strong field regime: when the particles become relativistic. They look at the curvature of light caused by gravitational effects, called  » gravitational lens ”: the black hole is so massive that the light does not follow a linear path when it passes near. This capability opens up a new avenue for testing general relativity in the strong-field regime, studying accretion and flow processes at the edge of a black hole, and probing the physical fundamental of black holes and the very existence of event horizons.