Gargantuan, Never-Before-Seen Radio Bubbles Discovered Near Milky Way's Supermassive Black Hole

Scientists have discovered two gargantuan radio bubbles at the center of the Milky Way, near the galaxy's supermassive black holes. These never-before-seen structures are believed to have formed during a "staggeringly powerful" eruption about seven million years ago, potentially from huge amounts of matter falling into the black hole, or from a massive burst of star formation.

The balloon-like structures stretch for 1,400 light-years, extending above and below the galactic plane—the plane on which most of the Milky Way's mass lies. "We were surprised," Fernando Camilo, from the South African Radio Astronomy Observatory, told Newsweek.

"Previous observations by other researchers with other telescopes of areas above (north of) the center of the galaxy had shown some radio features that we now realize are portions of the coherent bubble structure that we have discovered ... The origin of those radio features was not known. But especially below (south of) the center of the galaxy, nothing remarkable was known—and in fact some scientists wondered why there was something in the north but apparently nothing in the south."

Researchers were observing the center of the Milky Way with the MeerKAT radio telescope in South Africa. The environment of the center of our galaxy is vastly different to the rest of the Milky Way. In the middle lies a supermassive black hole, that compared to other galaxies, is relatively subdued. The region is also characterized by a massive cloud of dust, making it difficult to peer into with telescopes. Using MeerKAT, the team was able to look through the cloud to find radio signals that chart energetic regions of space.

Their findings, published in the journal Nature, showed two enormous radio bubbles almost identical in terms of size and shape. "The shape and symmetry of what we have observed strongly suggest that a staggeringly powerful event happened a few million years ago very near our galaxy's central black hole," co-author William Cotton, from the U.S. National Radio Astronomy Observatory, said in a statement.

Researchers propose two possible sources for the radio bubbles. They could be the result of mass falling into the supermassive black hole: "For instance, a star being torn to shreds, with some of its matter going into the black hole, never to come out, while other portions get flung outwards with enormous energy," Camilo said.

radio bubbles
Image showing the radio bubbles at the center of the Milky Way. The two bubbles extend above and below the galactic plane—which is marked by bright features. South African Radio Astronomy Observatory

It could also be the result of a period of rapid star formation. Some of these stars, Camilo explained, could have been very large and very short-lived, meaning they explode as supernova after just a few million years. "If you have lots—say, about 100—supernovae going off relatively quickly in a small region (near the galactic center—which ultimately happens because the black hole indirectly influences much that happens in the central regions of the Milky Way), then the huge amount of energy deposited into the surrounding regions of the interstellar medium will basically punch a hole into that medium and expand."

There is a cluster of young stars sitting about one light-year from the black hole that formed around the same time as the radio bubbles. "Is this a coincidence? We're not sure," said Camilo.

Camilo said that with either of these two options, you would end up with the inflation of a hot bubble like the ones observed. The bubbles were created an estimated seven million years ago and have been expanding ever since. "Within a few more million years they'll likely dissipate to the point where they'll be indistinguishable from the background radio emission from the central regions of the galaxy," he added.

Further observations of the bubbles with different telescopes, such as NASA's Chandra telescope, could provide more information on what produced them. "Right now, this discovery opens almost as many new questions as it helps to answer old puzzles," Camilo said.

Ziri Younsi, an astrophysicist at University College London, who was not involved in the study, said it was an exciting discovery—and one that could help our understanding of the center of our galaxy. The bubbles, he said, could be mini versions of 'Fermi bubbles'—mystery structures that span 50,000 light-years and originate in the Milky Way's center.

"This study is an important step forwards in understanding the connection of the central supermassive black hole with the galaxy itself," Younsi told Newsweek. "How does the supermassive black hole and the process by which it accretes (consumes) matter inject energy into the Milky Way, and how does it interact with the galaxy as a whole? This has important consequences for galaxy formation and growth, as well as for black hole evolution. Are such smaller bubbles episodic, and are they connected with the larger Fermi bubbles? These are interesting and important questions."

Astrophysicist Graham Woan, from Glasgow University, who was also not involved in the research, told Newsweek: "This is a whopping image ... and is a wonderful example of what can be achieved with more than a little image-stitching ingenuity and the recent generation of radio telescopes. MeerKAT is paving the way for a truly gigantic radio telescope to be constructed in the coming years—the Square Kilometre Array (SKA). We can look forward to many more fantastic views of the universe, like this one, when the SKA starts operation in the coming years."