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Sagittarius A*
Ever since Event Horizon Telescope (EHT) released the first image of a supermassive black hole at the center of Messier 87 (M87*), in 2019, I always wondered what the black hole at the galactic center of our own Milky Way will be like! And on May 12, 2022 we got to see the first picture of Sagittarius A* (Sgr A*), thanks to the Event Horizon Telescope Collaboration team.
Sagittarius A* #
Sagittarius A* is the astronomical radio source at the galactic center of Milky Way Galaxy. Two astrophysicists, Reinhard Genzel and Andrea Ghez were awarded 2020 Nobel Prize in Physics for their discovery that Sgr A* is a supermassive compact object, for which a black hole is the only currently known plausible explanation. And today's image from EHT provides the first direct visual evidence of it.
This breakthrough by the EHT Collaboration team follows the 2019 picture of M87* balck hole. While Sgr A* and M87* black holes look remarkably similar, their similarities end there. Sgr A* is more than a thousand times smaller and 1500 times less massive than M87* blackhole. This measurement and achievement were considerably harder than the previous one, even though Sgr A* is much closer at ~27,000 light years. “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity," said EHT Project Scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. The EHT team's results are being published today in a special issue of The Astrophysical Journal Letters.
EHT scientist Chi-kwan (‘CK’) Chan, from Steward Observatory and Department of Astronomy and the Data Science Institute of the University of Arizona, US, explains: “The gas in the vicinity of the black holes moves at the same speed — nearly as fast as light — around both Sgr A* and M87*. But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sgr A* it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT Collaboration was observing it — a bit like trying to take a clear picture of a puppy quickly chasing its tail.”
The researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A*. While M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sgr A*. The image of the Sgr A* black hole is an average of the different images the team extracted, finally revealing the giant lurking at the centre of our galaxy for the first time.
With two images of two very different sized black holes, the reseach now opens up to use new data to test theories and models over the coming months/years. Exciting times ahead!
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