Radio astronomers have imaged the supermassive black hole at the center of the Milky Way. It is only the second-ever direct image of a black hole, after the same team unveiled a historic picture of a more distant black hole in 2019.
The long-awaited results, presented today by the Event Horizon Telescope collaboration, show an image reminiscent of the earlier one: a ring of radiation surrounds a darker disk of precisely the size that was predicted from indirect observations and from Albert Einstein’s general theory of relativity.
“Today, right this moment, we have direct evidence that this object is a black hole,” said Sara, an astrophysicist at the Harvard and Smithsonian Center for Astrophysics in Cambridge, Massachusetts, at a press conference in Germany. The team published its results in a special issue of The Astrophysical Journal Letters1.
“We’ve been working on this for so long, every once in a while you have to pinch yourself and remember that this is the black hole at the center of our Universe,” said Event Horizon Telescope team member Katie, a computational-imaging researcher at the California Institute of Technology in Pasadena, at a press conference in Washington DC. “I mean, what’s cooler than seeing the black hole at the center of the Milky Way?”
Black-hole observations
During five nights in April 2017, the Event Horizon Telescope collaboration used eight observatories across the world to collect data from both the Milky Way’s black hole — called Sagittarius A*, after the constellation in which it is found — and M87*, the one at the center of the galaxy M87.
The observatory locations ranged from Spain to the South Pole and from Chile to Hawaii. They collected nearly 4 petabytes (4,000 terabytes) of data, which was too much to be sent over the Internet and had to be carried by aeroplane on hard disks.
The Event Horizon Telescope researchers unveiled their image of M87* in 2019, showing the first direct evidence of an event horizon, the spherical surface that shrouds a black hole’s interior.
But the Sagittarius A* data were more challenging to analyze. The two black holes have roughly the same apparent size in the sky, because M87* is nearly 2,000 times farther away but about 1,600 times larger. Any blobs of matter spiral around M87* are covering much larger distances — larger than Pluto’s orbit around the Sun — and the radiation they emit is essentially constant over short time scales. But Sagittarius A* can change quickly, even over the few hours that the Event Horizon Telescope observes it every day. “In M87*, we saw very little variation within a week,” says Heino, an astrophysicist at University in Nijmegen, the Netherlands, and a co-founder of the Event Horizon Telescope collaboration. “Sagittarius A* varies on time scales of 5 to 15 minutes.”
Because of this variability, the Event Horizon Telescope team generated not one image of Sagittarius A*, but thousands — and the image unveiled today is the result of a lot of processing. “By averaging them together, we are able to emphasize common features,” says Event Horizon Telescope member José Gómez, at the Institute of Astrophysics of Andalusia in Granada, Spain.
In addition to showing a ring of radiation around a darker disk, the resulting image contained three brighter ‘knots’. “We see knots in all the images we created,” but each had the knots in different places. The averaged knots that appear in the image are probably of the interferometry technique used by the Event Horizon Telescope, she adds. It reconstructs images from an idealized Earth-sized radio dish — but one in which only tiny shards of the dish are able to take data at any given time.
The appearance is different from that of M87*, for which the brighter region in the image had more of a half-moon shape, which could indicate a denser blob of matter being accelerated along the direction of the line of sight.
The project’s next aim is to generate a movie of the black hole to learn more about its physical properties, an astrophysicist at the University of Arizona in Tucson, told reporters at the Washington, DC press conference.
The Event Horizon Telescope team conducted supercomputer simulations to compare with their data and concluded that Sagittarius A* is probably rotating anticlockwise along an axis that roughly points along the line of sight to Earth, said Gómez.
“What blows my mind is that we’re seeing it face-on,” says Regina Caputo, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. NASA’s Fermi Gamma-ray Space Telescope, which Caputo works with, had previously detected giant glowing features above and below the center of the galaxy, which could have been produced by Sagittarius A* during periods of intense activity in the past. But those features, known as Fermi bubbles, seem to require matter to swirl around the black hole edge-on, rather than face-on, as seen from Earth.
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