Astronomers say they've finally seen, for the first time, evidence of a powerful wind blowing from the supermassive black hole at the centre of our Galaxy.
The Milky Way's central supermassive black hole is known as Sagittarius A*, and the team behind the study say they've uncovered the most detailed view yet of how the black hole interacts with and transforms its surrounding environment.
They say the discovery could provide an answer to one of the biggest mysteries in space science.
Black holes are not just suckers
Black holes may have a reputation for being cosmic vacuum cleaners, sucking up anything that strays to close to their path.
But it's well established that black holes also fire out powerful jets of material into space.
Scientists say even a small amount of gas falling into a black hole should generate enough energy to push material outwards.
That cosmic 'wind' had never been detected coming from Sagittarius A*, but now astronomers say they've found evidence of it happening.
Read more
"Unless a black hole exists in a perfect vacuum, it must blow a wind somehow,” says Mark Gorski of Northwestern University in Illinois, USA, who co-led the study.
"And there is no perfect vacuum in the Universe. With new observations, this is the first time we’ve had a clean enough view to see the wind’s imprint.
"We looked at the data and said, 'There it is. There is the thing that everybody’s been looking for for 50 years'."
"We were the first to show that molecular gas very, very close to the black hole is feeding it," says Elena Murchikova, who co-led the study.
"The wind is not powerful, and its direction probably wanders with time. It shows that our black hole is not unique, and our place in the universe is not unique."
Finding the wind at the Galaxy's heart
Why do black holes spew out matter into the cosmos?
Theoretical physicists say all actively 'feeding' black holes should launch powerful outflows, or jets, into space.
This happens because, as material spirals into a black hole, it moves faster and faster and eventually reaches speeds close to the speed of light.
That creates so much energy, it flings some of the hot, fast-moving material out into space in the form of winds or jets.
Astronomers had spotted evidence of past eruptions from Sagittarius A*, but had never seen it happening in real time.
That, say the team behind this study, is likely because our Galaxy's central supermassive black hole is going through a quiet phase and is also difficult to see.
"To observe our own black hole, we have to look through the plane of our galaxy," says Murchikova.
"That means we have to peer through gas, dust and ionised structures – and you can’t really see through all of that easily."
A cosmic cone
The team used five years of deep-sky observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) observatory in Chile.
They were able to build the sharpest image ever created of cold molecular gas surrounding the black hole.
The image shows gas within about three lightyears of the black hole – a stone's throw in cosmic terms. Then, with clever image processing, they were able to remove the black hole’s bright radiowave signals.
The team say their image is 100 times deeper and 80 times sharper than previous maps of the region. It's even revealed structures that were invisible in previous observations.
What's more, it shows a huge cone-shaped cavity that'sdevoid of cold molecular gas.
According to the team, only hot, energetic wind blowing from the black hole could have created the cavity.
"If you blow hot material from the black hole, it’s not going to want to exit with the cold material," Gorski says.
"It’s either going to push the cold material out or heat it up. And, if it’s too hot, you will no longer see the cold gas.
“It’s a huge absence of material. We calculated how much energy was needed to create this cavity.
"It is more than can be provided by the stars in that area. Basically, there has to be input from the supermassive black hole.
"And, if you follow the shape of the cone, it’s pointed directly at the black hole."
The team used data from NASA’s Chandra X-ray Observatory to help confirm their results.
Chandra images showed bright X-ray emissions in the exact same region. And in the same location as the bright X-rays, cone-shaped hollow region could be seen where cold gas was missing.
"Exceptional claims require exceptional evidence," Gorski says.
"We wanted to make sure that we weren’t just looking at some sort of imaging artifact. Then, the X-ray image from Chandra just slotted in perfectly. The molecular features lined up."
"When you find something no one has seen before, the first thought that runs through your mind is not 'Oh my god, we made a discovery,'" Murchikova says.
"It’s 'Oh my god, what’s wrong with my analysis?' But when we overlaid our image with the X-ray image, it started to make sense."
Our quiet, central black hole
The astrophysicists estimate the wind has been active for at least 20,000 years.
And they say the discovery confirms that Sagittarius A* is quiet compared to other galaxies’ central supermassive black holes.
"The majority of other galaxies spend most of their lives in a state where they are not particularly active," Murchikova says.
"But we can only see them when they are in a fireworks stage. It is very attractive to study black holes when they are in the fireworks stage, but that’s not actually their dominant state.
"Sgr A* finally gives us a window into the life of a black hole in this quiet state."
