The hunt is over.
After more than 50 years of searching, astrophysicists at Northwestern University have finally discovered evidence of a powerful wind blowing from the Milky Way's central supermassive black hole, Sagittarius A* (Sgr A*).
According to theoretical physics and a long-accepted understanding of galaxies evolution, as black holes consume materials, they should produce wind or jets. Even a small amount of gas falling into a black hole should generate enough energy to push material outwards. Without wind, Sgr A* would be a unique outlier.
But, until now, no one could find it.
By providing the most detailed view yet of how Sgr A* interacts with and transforms its surrounding environment, the scientists resolved one of the longest-standing mysteries in astronomy. It also opens a new window into the physics at play in the center of the Milky Way.
The study will be published on Thursday (June 4) in The Astrophysical Journal Letters.
"Unless a black hole exists in a perfect vacuum, it must blow a wind somehow," said Northwestern's Mark Gorski , 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," said Elena Murchikova , who co-led the study with Gorski. "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."
Focused on the evolution of galaxies, Gorski is a research assistant professor at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). An expert on black hole astrophysics, Murchikova is an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences and member of CIERA.
Elusive wind at the galaxy's heart
Although black holes are infamous for swallowing anything that ventures too close, they don't just pull matter in. They also push material out. For decades, theorists have predicted that all actively feeding black holes launch powerful outflows. As material spirals inward toward a black hole, it moves faster and faster — until it reaches close to the speed of light. This creates enough energy and pressure to fling some of the hot, fast-moving material outward in the form of winds or jets.
While astronomers have spotted evidence of past eruptions from Sgr A*, they struggled to detect currently occurring outflows. The Northwestern team says this is likely because Sgr A* is in a quieter phase and just incredibly difficult to see.
"To observe our own black hole, we have to look through the plane of our galaxy," Murchikova said. "That means we have to peer through gas, dust and ionized structures, and you can't really see through all of that easily."
A cone-shaped cavity
Now, with new tools and observations, the team finally was able to take a closer look. Using five years of extraordinarily deep observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) radio telescopes in Chile, Gorski and Murchikova constructed the sharpest image ever devised of cold molecular gas surrounding the black hole.
The image reflected the gas located incredibly close to Sgr A* — within just one parsec (or about three light-years) of the black hole. Then, the duo applied a calibration method to remove the black hole's bright radio signals. The resulting image is 100 times deeper and 80 times sharper than previous maps of the region. With this level of detail, it revealed structures that were completely invisible in previous observations.
But one newly revealed, unmistakable feature left Gorski and Murchikova gobsmacked. A vast, cone-shaped cavity — nearly one parsec long and 45 degrees wide — was devoid of cold molecular gas. According to the researchers, only hot, energetic wind blowing from Sgr A* could have created this hollowed-out region. Wherever the hot wind travels, it either sweeps cold gas away or heats it up.
"If you blow hot material from the black hole, it's not going to want to exist with the cold material," Gorski said. "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."
Exceptional claims, exceptional evidence
While stars, too, create winds, stellar winds are not powerful enough to carve out a cleanly swept region of this size. Even the combined power of all the nearby stars falls short.
"It's a huge absence of material," Gorski said. "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."
Before declaring they solved a long-standing mystery, Gorski and Murchikova continued to analyze data to further confirm their results. NASA's Chandra X-ray Observatory previously pinpointed bright X-ray emissions in the exact same region. In the same location as the bright X-rays, a hollow, cone-shaped region appeared where cold gas was missing.
"Exceptional claims require exceptional evidence," Gorski said. "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 that no one has seen before, the first thought that runs through your mind is not 'Oh my god, we made a discovery,'" Murchikova said. "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."
A quiet phase of life
Based on how far its effects extend into a nearby stream of ionized gas, the astrophysicists estimate the wind has been active for at least 20,000 years. The discovery also confirms that Sgr A* is relatively 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 said. "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."
