And two! More than 300 researchers from 80 institutes united in the ambitious Event Horizon Telescope (EHT) radio astronomy program have reproduced a major scientific feat: taking a photograph – or rather an image – of a supermassive black hole. The first was that of M87*, the black hole at the center of the Messier 87 Galaxy, 55 million light-years from us, revealed on April 10, 2019. This time, the object is much closer, since it is “our” supermassive black hole, Sagittarius A *, located only 27,000 light years from Earth, in the center of the Milky Way. This is new irrefutable proof of the existence of these giants, predicted by the laws of general relativity of Albert Einstein, but which remain the most mysterious objects in the universe, because the most difficult to observe.
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And for good reason: these gravitational monsters distort not only space, but also time, which slows down around them. Their surface is an immaterial border, poetically called “horizon of events”, from which nothing escapes, not even light. These celestial objects are completely invisible. And yet, it is not impossible to guess them, because these space monsters attract towards them all kinds of matter, including immense gaseous clouds which form “discs of accretion”. The latter, extremely hot and luminous, emit a characteristic radiation which betrays the position of the black holes. It is thanks to these gases that the international consortium of astronomers of the EHT was able to see M87* and, now, Sagittarius A*.
Both images of supermassive black holes. M87* (left) and Sagittarius A* (right). An “astonishing” similarity, explain the researchers.
EHT/ESO
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“We discovered a ring of gas similar to that of M87 *, although more imposing, which was expected, explains Vincent Piétu, astronomer and researcher at the Institute of Millimetric Radio Astronomy (Iram) in Grenoble, which is located in the heart of the EHT project We are all very happy with this discovery and this announcement, which confirms that the object at the center of our galaxy is very compact and that it is indeed a black hole, which was theoretically proven, but that we weren’t 100% sure about.” Thanks to this confirmation, researchers will now be able to carry out more precise tests on the stars that orbit not far from the supermassive black hole, and thus improve our knowledge of the center of our galaxy, and above all test more precisely Einstein’s general relativity or even , one day, push back the limits of science? “Everyone wanted to track down black holes, one of the greatest mysteries of physics. This image has taken us to the limits of our knowledge. As crazy as it may seem: it is at the edge of black holes that our possibilities of measurement and study, and we do not know if we will one day be able to cross this limit”, writes the astrophysicist Heino Falcke, one of the founders of the EHT project, in his book Light in the dark (ed. Buchet-Chastel), published in January 2022.
Sagittarius A* was well hidden
The EHT researchers are thus filling a (small) frustration of astronomy enthusiasts, who were all waiting for the snapshot of Sagittarius A* in 2019, and had been surprised to discover that of M87*. In reality, the data allowing the snapshot of Sagittarius A* were indeed collected in 2017, at the same time as those of M87*. But their analysis turned out to be more complex. “Sagittarius A* is closer, but it is also 1,500 times less massive than M87* – 4.14 million solar masses against 6 billion -, but its lower mass means that the characteristic time around it is 1,000 times faster”, explains Vincent Piétu. In other words, the environment around Sagittarius A* being much more moving than around M87*, obtaining a clear and precise image while the observation time is only 8 to 10 hours required a lot of corrective calculations.
<img xmlns:xlink="http://www.w3.org/1999/xlink" data-optimumx="1.5" data-sizes="auto" alt="Le radiotélescope de 30-mètres de l'IRAM, situé a plus de 2 800 mètres au Pico Veleta dans la Sierra Nevada espagnole. C'est le radiotélescope à antenne unique le plus sensible au monde." class="article__item–img loading_medium lazyload lazy img_resp_full media_content_full" src="data:image/svg+xml;utf8,IRAM’s 30-meter radio telescope, located over 2,800 meters at Pico Veleta in the Spanish Sierra Nevada. It is the most sensitive single-antenna radio telescope in the world.
IRAM/EHT
Also, since Sagittarius A* sits in the middle of our galaxy, there are more obstacles in the line of sight between Earth and it, including plasma clouds. “They screened the light we wanted to see, and although the effect was modest, it made it difficult to process the information because we wanted to have a high degree of certainty, so we took our time and did all the necessary verifications, details the scientist. So we started with the easiest, M87*, before tackling Sagittarius A*. And then the Covid crisis hit and also contributed to delaying the project”. But this Thursday, May 12, EHT researchers can finally reveal the image of the supermassive black hole, as well as the publication of more than ten major scientific articles, including six “collaboration papers”, four “official papers” and “relative papers”. “I printed them all and placed them on my desk, I can tell you that the whole thing forms a nice pile of scientific papers!”, laughs Vincent Piétu.
The EHT project, quintessence of international collaboration
To achieve these feats, the approximately 300 researchers used a network of 11 telescopes spread across the four corners of the planet, even if not all of them took part in the two observations. Two, including the one on the Bure plateau in France, didn’t take part in observations until after 2019. “And the South Pole telescope couldn’t observe M87*, but it did see Sagittarius A*, from the same way, that of Greenland only participated in the observations of M87 *”, adds the astronomer. But by combining data from eight different observatories, the researchers still simulated the power of a single telescope with a diameter of 10,000 kilometers, more than the distance between Paris and Tokyo, allowing them to reach a degree unprecedented precision. It is thanks to this technique that the EHT project was able to see supermassive black holes at the center of the Messier galaxy and ours. But before achieving this, the researchers had to arm themselves with patience. Firstly because these various radio astronomy stations are in high demand by the astronomical community, but also because the meteorological conditions had to be optimal – a sky without rain and without clouds – and this simultaneously for all the telescopes.
<img xmlns:xlink="http://www.w3.org/1999/xlink" data-optimumx="1.5" data-sizes="auto" alt="Le projet Event Horizon Telescope (EHT) associe des télescopes du monde entier – de l'Europe jusqu'au pôle sud, en passant par le Chili et Hawaii. Carte des télescope qui ont été utilisés pour les observations entre 2017 et 2020." class="article__item–img loading_medium lazyload lazy img_resp_full media_content_full" src="data:image/svg+xml;utf8,The Event Horizon Telescope (EHT) project brings together telescopes from all over the world – from Europe to the South Pole, via Chile and Hawaii. Map of the telescopes that were used for observations between 2017 and 2020.
NRAO
It was then necessary to ensure that the signals received by each of the antennas were synchronized to within a tenth of a billionth of a second. The researchers made sure of this thanks to atomic clocks installed in each observatory, but also by calculating the geographical position of all the stations, to within a fraction of a millimeter, which required taking into account parameters as fine as the continental drift… The researchers also had to collect the largest mass of data collected in scientific history, which is measured in petabytes – millions of gigabytes – saved on thousands of hard drives. Finally, they analyzed them using supercomputers that took several months to independently process the mountain of data and finally acquire the first pixels. “This work shows the value of quasi-international collaboration, because if we – countries and researchers – had not pooled our resources, we would never have achieved such a result”, emphasizes Vincent Piétu. A message that is all the more important in this period of war in Ukraine and ever-increasing international tensions.
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The work of the EHT has only just begun, since the next stage will consist of “combatting the movements of the images obtained”, and producing, in the medium term, a ten-hour film allowing us to see the evolution of Sagittarius A*, or rather accretion disks blazing around it. Before that, astronomers will also try to publish an even more precise image of M87* which will show the jets of gas ejected by the black hole.
Victor Garcia
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