The Milky Way Has Giant Bubbles at Its Center
Astronomers have found radio-emitting structures jutting out from our galaxy’s black hole.
Farhad Yusef-Zadeh was observing the center of the Milky Way galaxy in radio waves, looking for the presence of faint stars, when he saw it: a spindly structure giving off its own radio emissions. The filament-like feature was probably a glitch in the telescope, or something clouding the field of view, he decided. It shouldn’t be here, he thought, and stripped it out of his data.
But the mystery filament kept showing up, and soon Yusef-Zadeh found others. What the astronomer had mistaken for an imperfection turned out to be an entire population of cosmic structures at the heart of the galaxy.
More than 100 filaments have been detected since Yusef-Zadeh’s first encounter in the early 1980s. Astronomers can’t completely explain them, but they have given them familiar labels, naming them after the earthly things they resemble: the pelican, the mouse, the snake. The menagerie of filaments is clustered around the supermassive black hole at the center of our galaxy. “They haven’t been found elsewhere,” says Yusef-Zadeh, a physics and astronomy professor at Northwestern University.
Their origins remained a mystery, too, until now.
New observations of the galactic center have revealed a pair of giant bubbles at the center of the Milky Way that give off radio emissions, according to recent research published in Nature. The bubbles stretch outward from the black hole and extend into space in opposite directions. The billowy lobes resemble the two halves of an hourglass, with the black hole nestled at its waist. And the filaments that Yusef-Zadeh discovered all those years ago are encased within.
These bubbles are big. Top to bottom, the cosmic hourglass measures 1,400 light-years, a distance that, if converted into miles, comprises of a dizzying number of zeros; one light-year—the distance that light covers in an Earth year—is about 6 trillion miles. The black hole, by comparison, is a pinprick of light.
The discovery suggests that the sinuous filaments arose as part of a larger structure. “We’ve long thought that this was the case, but we haven’t been able to image the proof,” says Cornelia Lang, an astronomer at the University of Iowa who studies these filaments, and who was not involved in the bubble research.
The new observations come from the MeerKAT telescope in South Africa, an array of dozens of dishes that work together to generate a large field of view. The facility, which began operations last year, is located in one of the best places on the planet to study the heart of the Milky Way; the galactic center passes right overhead and remains observable for hours.
But it’s not a straight shot. Our solar system resides near one of the Milky Way’s shimmery spirals, and there are about 25,000 light-years of gas, dust, and other cosmic matter sitting between us and the galactic center. To observe that faraway place, astronomers must observe in forms of electromagnetic radiation other than visible light, like radio. “We have to piece together a picture of the center of the galaxy using wavelengths that are not the kind that our eye sees,” Lang says.
The astronomers behind the bubble discovery looked for a specific kind of radio emission generated in turbulent regions of space, where electrons move at close to the speed of light and bounce around magnetic fields. As the charged particles zoom, they give off radio waves that can illuminate cosmic structures in the vicinity. By capturing this radiation, astronomers have illuminated the contours of the bulbs and the structures they contain.
The bubbles look like a carefully spun, delicate work of interstellar art. But they are the aftermath of a violent, cosmic cataclysm that unfolded millions of years ago.
“Something happened, in a very short period of time, a few million years ago at the center of the galaxy,” says Fernando Camilo, the chief scientist at the South African Radio Astronomy Observatory and one of the members of the international team responsible for the discovery.
Camilo and other astronomers are considering a couple of explanations. A flurry of dying stars at the center of the galaxy might have infused the medium with enormous amounts of energy as they exploded. Or it could be that the black hole experienced a flare-up, as black holes around the universe have been known to do. Sometimes, black holes consume nearby stellar material so quickly that they end up regurgitating some of it. The result is two luminous jets of radiation that can outshine entire galaxies. Our supermassive black hole is in a quiet chapter of its life, but astronomers suspect that it has previously experienced this active phase.
Whatever happened, it was powerful enough to create a burst of energy that, as the researchers put it, literally punched through the interstellar medium. “As it punches through the medium, it’s sweeping the material of the medium with it,” says Yusef-Zadeh, one of the members of the team. “Eventually, it reaches a point where it stalls.”
The ancient explosion inflated the bubbles and, as they expanded, excited the electrons that, together with nearby magnetic fields, produce radio emissions we can detect all the way from here. “This is the most exquisite radio map of the galactic center that has ever been published,” says Grant Tremblay, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, who was not involved in the work.
The heart of the galaxy is home to other bubbles, recorded in other wavelengths. The Fermi bubbles, named for the 20th-century scientist who studied high-energy physics, are even larger, stretching about 25,000 light-years above and below the galactic center. Astronomers discovered them nearly a decade ago with a space telescope designed to detect gamma rays. And they’re still trying to understand them. Camilo speculates that perhaps the Fermi bubbles might be the dumping grounds of eons of many cosmic explosions—a larger, older version of the radio-emitting bubbles his team found.
There may be more bubbles, more pelican-shaped mysteries, lurking in this panoramic view of the center of the galaxy. As telescopes collect more data—if new, more advanced facilities come online—the portrait will become sharper. And astronomers should be prepared to find something so strange that they’re tempted to chuck them out of the data. “I remember vividly that I spent quite a bit of time to get rid of things that I would not have really done if I knew these structures existed,” Yusef-Zadeh says. “It was just so weird.”