An image of the supermassive black hole at the center of the Milky Way has been captured, giving the first direct glimpse of the turbulent heart of our galaxy.
The black hole itself, known as Sagittarius A*, pronounced “Sagittarius A-Star,” cannot be seen because no light or matter can escape its gravitational grip. But its shadow is traced out by a glowing, fuzzy ring of light and matter that is swirling on the precipice at close to the speed of light before its eventual plunge into oblivion.
The image was captured by the Event Horizon telescope (EHT), a network of eight radio telescopes spanning locations from Antarctica to Spain and Chile, which produced the first image of a black hole, in a galaxy called Messier 87, in 2019.
Prof Sera Markoff, an astrophysicist at the University of Amsterdam and co-chair of the EHT Science Council, said: “The Milky Way’s black hole was our main target, it’s our closest supermassive black hole and it’s the reason we set out to do this thing in the first place. It’s been an 100-year search for these things and so scientifically it’s a huge deal.”
The image provides compelling proof that there is a black hole at the center of the Milky Way, which had been the working assumption of mainstream astronomy. But a minority of scientists had continued to speculate about the possibility of other exotic objects such as boson stars or clumps of dark matter.
“I’m personally happy about the fact it really drills home the fact that there is definitely a black hole at the center of our galaxy,” said Dr Ziri Younsi, a member of the EHT collaboration who is based at University College London. “It’s a turbulent, chaotic and quite violent environment. It made me think, ‘Wow, we’re quite lucky to live at the edge of the galaxy actually.’”
To the untrained eye, the latest image might appear similar to that of M87, which is 55m light years from Earth, but the observations are already giving entirely new scientific insights. And, Younsi said, there was an emotional, as well as purely scientific, value in finally seeing the enigmatic object about which our home galaxy revolves. “It’s another doughnut, but it’s our doughnut,” he said.
A resolution the equivalent of seeing a bagel on the moon was required to bring it into focus.
Despite being local in astronomical terms (still 26,000 light years away) observing SgrA* turned out to be more challenging than anticipated and the team has spent five years analyzing data acquired during fortuitously clear skies across several continents in April 2017. Sagittarius A* is more than a thousand times smaller and less massive than M87*, meaning a resolution the equivalent of seeing a bagel on the moon was required to bring it into focus.
Its size means dust and gas is orbiting it in a matter of minutes, rather than weeks, so the image was constantly changing from one observation to the next. Markoff compared the challenge to trying to capture a puppy chasing its tail using a camera with a slow shutter speed. And the scientists had to peer through the galactic plain, meaning radiation from all the intervening stars had to be filtered out. Some combination of these factors – and possibly some extreme black hole phenomenon – explain the bright blobs in the image.
“We didn’t anticipate how evasive and elusive it would be,” said Younsi. “It was really a tough picture to take – it’s hard to overstate that.”
Four million times more massive than our Sun.
The EHT picks up radiation emitted by particles within the accretion disc that are heated to billions of degrees as they orbit the black hole at close to the speed of light, before vanishing into the central vortex. The blotchy halo in the image shows light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun.
The latest observations are already giving intriguing hints about the nature of our own black hole. Simulations based on the data hint that our black hole’s angle of rotation is not neatly aligned with the galactic plain, but is off-kilter by about 30 degrees. The observations also suggest that SgrA* is in a dormant state, in contrast with some black holes, including M87, which feature vast, powerful jets that blast light and matter from the black hole’s poles into intergalactic space. “If a big star fell in, which would happen every 10,000 years, that would wake it up for a short amount of time and we’d see things brighten up,” said Markoff.
Ultimately, scientists hope that observing these competing processes in black holes – gobbling up nearby material versus blasting it outwards into space – could help answer a chicken-and-egg style question about the evolution of galaxies.
“It’s an open question in galactic formation and evolution. We don’t know which came first, the galaxy or black hole,” said Prof Carole Mundell, an astrophysicist at the University of Bath who is not part of the EHT collaboration.
“From the technology perspective it’s mind-blowing that we can do this,” she said of the latest images.
The EHT team’s results are being published on Thursday in a special issue of the Astrophysical Journal Letters.11