An image of the supermassive black hole at the heart of the Milky Way has been captured, giving the first direct view of the ‘gentle giant’ at the center of our galaxy.
The black hole itself, known as Sagittarius A*, cannot be seen because no light or matter can escape its gravitational grip. But its shadow is outlined by a glowing, fuzzy ring of light and matter that whirls across the abyss at nearly the speed of light.
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, astrophysicist at the University of Amsterdam and co-chair of the EHT Science Council, said: “The Milky Way black hole was our main target, it is our closest supermassive black hole and that is why we decided to do it have thing in the first place. It’s been a 100-year quest for these things, so scientifically it’s a huge thing.”
The image provides compelling evidence that there is a black hole at the center of the Milky Way, which has been the working assumption of mainstream astronomy. A minority of scientists had continued to speculate about the possibility of other exotic objects such as boson stars or clumps of dark matter.
“Personally, I’m excited about the fact that it makes it really clear that there is definitely a black hole at the center of our galaxy,” said Dr. Ziri Younsi, a member of the EHT collaboration based at University College London.
To the untrained eye, the latest image might appear roughly similar to that of black hole M87*, but the two objects are extremely different, according to the EHT team.
Sagittarius A* uses only a trickle of material, in contrast to the typical portrayal of black holes as violent, ravenous monsters of the cosmos. “If SgrA* were human, it would only eat a single grain of rice every million years,” said Michael Johnson of the Harvard-Smithsonian Center for Astrophysics.
In contrast, M87* is one of the largest black holes in the universe, and features huge, powerful jets that hurl light and matter from its poles into intergalactic space.
“Sgr A* gives us a glimpse into the much more common state of black holes: calm and still,” Johnson said. “[It] is exciting because it is common.”
The latest observations also appear to show that our black hole’s rotation angle is not neatly aligned with the galactic plane, but is out of balance by about 30 degrees, and suggest spectacular magnetic activity similar to that observed in the Sun’s atmosphere . Beyond science, astronomers acknowledged an emotional connection to finally seeing the enigmatic object our home galaxy revolves around.
“It’s a different donut, but it’s our donut,” Younsi said.
Although astronomically local at 26,000 light-years away, observing SgrA* proved more difficult than expected. The team spent five years analyzing data collected in April 2017 under randomly clear skies across multiple continents.
Sagittarius A* is relatively small, meaning the dust and gas in its accretion disk orbits in minutes instead of weeks, creating a moving target from one observation to the next. Markoff likened the observations to trying to photograph a puppy chasing its tail with a slow shutter speed camera. The scientists also had to look through the galactic plane and filter out intervening stars and dust clouds from their images. A combination of these factors – and possibly an extreme black hole phenomenon – explains the bright blobs in the image.
“We didn’t expect how evasive and elusive it would be,” Younsi said. “It was really a difficult picture to take. It’s hard to overdo it.”
The EHT captures radiation emitted by particles within the accretion disk, which are heated to billions of degrees as they orbit the black hole before plunging into the central vortex. The speckled halo in the image shows light being bent by the strong gravity of the black hole, which is 4 m times more massive than our Sun’s.
Ultimately, scientists hope that observing a range of black holes – fairly quiescent like our own and turbulent giants like M87* – could help answer a chicken-and-egg question about galaxy evolution.
“This is an open question in galactic formation and evolution. We don’t know which came first, the galaxy or the black hole,” said Prof Carole Mundell, an astrophysicist at the University of Bath who is not part of the EHT collaboration.
“From a technological perspective, it’s mind-blowing that we can do this,” she said of the latest images.
The EHT team’s findings will be published in a special issue of the Astrophysical Journal Letters on Thursday.