Black hole at the centre of our Galaxy imaged for the first time

Radio astronomers have imaged the super massive black hole at the centre 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 (EHT) collaboration, show an image reminiscent of the earlier one, with a ring of radiation surrounding a darker disk of precisely the size that was predicted from indirect observations and from Albert Einstein’s theory of gravity.

“Today, right this moment, we have direct evidence that this object is a black hole,” said astrophysicist Sara Issaoun of the Harvard Smithsonian Center for Astrophysics at a press conference in Garching, Germany.

“We’ve been working on this for so long, every once and a while you have to pinch yourself and remember that this is the black hole at the centre of our Universe,” said computational-imaging researcher and former EHT team member Katie Bouman at a press conference in Washington, DC. “I mean, what’s more cool than seeing the black hole at the centre of the Milky Way?”

During five nights in April 2017, the EHT collaboration used eight different 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 — as well as the one at the centre of the galaxy M87, called M87*.

The observing locations ranged from Spain to the South Pole and from Chile to Hawaii, and added up to nearly four 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 EHT 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 analyse. The two black holes have roughly the same apparent size in the sky, because M87* is nearly 2,000 times farther away but also roughly 1,600 times larger. This also means that any blobs of matter that spiral around M87* are covering much larger distances — larger than the orbit of Pluto 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 the EHT observes it every day. “In M87* we saw very little variation within a week,” says Heino Falcke, an astrophysicist at Radboud University in Nijmegen, the Netherlands and a co-founder of the EHT collaboration. “Sagittarius A* varies on time scales of 5 to 15 minutes.”

Because of this variability, the EHT 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,” said EHT member José Gómez of the Andalusian Institute of Astrophysics in Granada, Spain. The next aim of the project is to generate a movie of the black hole to learn more about its physical properties, said Feryal Özel, an astrophysicist at the University of Arizona in Tucson.

The EHT team conducted supercomputer simulations to compare with their data, and concluded that Sagittarius A* is probably rotating along an axis that roughly points along the line of sight to Earth. The direction of that rotation is anticlockwise, Gómez said.

“What blows my mind is that we’re seeing it face-on,” says Regina Caputo, an astrophysicist at NASA–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 centre 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, would seem to require matter to swirl around the black hole edge-on as seen from Earth, rather than face-on.

The first hints of the existence of Sagittarius A* were seen in the 1970s, when radio astronomers discovered a seemingly pointlike radio source in the central region of the Galaxy.

The source turned out to be unusually dim, dimmer than an average star. Still, decades-long observations of the motions of nearby stars revealed that the object was extremely massive. The most recent ones have measured it to be 4.15 million times the mass of the Sun, give or take 0.3%. These calculations, done by tracking how stars orbit Sagittarius A*, provided strong evidence that the radio source is so massive and dense that it could be nothing else than a black hole, and earned Andrea Ghez and Reinhard Genzel a share of the 2020 Nobel Prize in Physics. (The EHT image shows that the black hole weighs around 4 million solar masses, which is consistent with those earlier estimates, although not as precise.)

Sagittarius A* is practically invisible to optical telescopes, because of the dust and gas on the galactic disk. But beginning in the 1990s, Falcke and others realized that the shadow of the black hole might be just large enough to be imaged with short radio waves, which can pierce that veil. But to do so, researchers calculated, would require a telescope the size of Earth. Fortunately, the technique called interferometry could help. It involves pointing multiple, faraway telescopes at the same object simultaneously. Effectively, the telescopes work as if they were shards of one big dish.

The first attempts to observe Sagittarius A* with interferometry used relatively long, 7 millimetre radio waves, and observatories a few thousand kilometres apart. All astronomers could see was a blurred-out spot.

Teams worldwide then refined their techniques, and retrofitted some major observatories so that they could add them to the network. In particular, a group led by Shep Doeleman of Harvard University in Cambridge, Massachusetts, adapted the South Pole Telescope and the US$1.4-billion Atacama Large Millimetre/submillimetre Array (ALMA) in Chile to do the work. In 2008, Doeleman’s team also conducted the first observations at the more technically challenging 1.3 millimetre wavelength.

Then in 2015, groups joined forces as the EHT collaboration. Their 2017 observation campaign was the first one to span distances long enough to resolve details the size of Sagittarius A*.

The EHT collected more data in 2018 but cancelled their planned observation campaigns in 2019 and 2020. They resumed observations in 2021 and 2022, with an improved network and more sophisticated instruments.

Remo Tilanus, an EHT member at the University of Arizona in Tucson, says that the team’s latest observations, in March, recorded signals at twice the rate as in 2017 — which should help to increase the resolution of the resulting images.

Researchers also hope to find out whether Sagittarius A* has jets. Many black holes, including M87*, display two beams of rapidly matter shooting out in opposite directions, presumably as a result of the intense heating of the in-falling gas. Sagittarius A* might have had large jets in the past — as heated clouds of matter above and below the galactic centre suggest. Its jets would now be much weaker, but their presence could still reveal important details about our Galaxy’s history.

“These jets can inhibit or induce star formation, they can move the chemical elements around,” and affect the evolution of an entire galaxy, says Falcke. “And we’re now looking at where it’s happening.”