A Pair of Large, Mysterious Radio Bubbles Sits at the Center of Our Galaxy

Several million years ago, a mysterious, violent event took place in the Milky Way that has left behind a distinct radio imprint in the galaxy’s center. Standing hundreds of light-years tall, a massive hourglass-like structure made up of a pair of twin radio wave-emitting bubbles makes all other structures in the center of the Milky Way look tiny in comparison.

The mysterious pair of bubbles, one of the largest structures ever observed in the Milky Way, was detected by South Africa’s MeerKAT radio telescope, and scientists aren’t exactly sure what may have caused it.

But they have some ideas.

In a paper published in Nature on Wednesday, a team of scientists suggests that the giant radio bubbles were the result of a violent eruption that went off in opposite directions over a short period of time near the supermassive black hole at the Milky Way’s center.

The study’s lead author, Ian Heywood, Ph.D., a research fellow at the University of Oxford, suspects that this may have occurred when the black hole was on a “feeding frenzy” that resulted in the outbursts.

“The center of our galaxy is relatively calm when compared to other galaxies with very active central black holes,” Heywood said in a statement. “Even so, the Milky Way’s central black hole can — from time to time — become uncharacteristically active, flaring up as it periodically devours massive clumps of dust and gas.”

The eruption could have also been caused when interstellar gas fell into the black hole, or when shockwaves created by large numbers of stars formed around the same time, Heywood and his team wrote in the paper.

Indications of this eruption were first discovered in the early 1980s by Farhad Yusef-Zadeh, Ph.D., from Northwestern University’s physics and astronomy department, who was also a co-author on this new study.

Back then, Yusef-Zadeh observed large, organized magnetic filaments, or strings of dark matter, that mark the boundaries of cosmic voids, at the center of the Milky Way. Ever since, he has been puzzled by their origin.

However, the recent observations showed most of the filaments confined within the radio bubbles.

“The radio bubbles discovered with MeerKAT now shed light on the origin of the filaments,” Yusef-Zadeh said in the statement.

Before, it was harder to observe that region of the Milky Way due to bright radio signals emitted from near its center, but the telescope used for this study captured the radio waves from the bubble-like structure despite the interference from its surroundings.

“These enormous bubbles have until now been hidden by the glare of extremely bright radio emission from the center of the galaxy,” Fernando Camilo, Ph.D., of the South African Radio Astronomy Observatory in Cape Town and a co-author of the paper, said in the statement.

“Teasing out the bubbles from the background noise was a technical tour de force.”

> Abstract: The Galactic Centre contains a supermassive black hole with a mass of four million Suns within an environment that differs markedly from that of the Galactic disk. Although the black hole is essentially quiescent in the broader context of active galactic nuclei, X-ray observations have provided evidence for energetic outbursts from its surroundings. Also, although the levels of star formation in the Galactic Centre have been approximately constant over the past few hundred million years, there is evidence of increased short-duration bursts, strongly influenced by the interaction of the black hole with the enhanced gas density present within the ring-like central molecular zone at Galactic longitude |l| < 0.7 degrees and latitude |b| < 0.2 degrees. The inner 200-parsec region is characterized by large amounts of warm molecular gas, a high cosmic-ray ionization rate, unusual gas chemistry, enhanced synchrotron emission, and a multitude of radio-emitting magnetized filaments, the origin of which has not been established. Here we report radio imaging that reveals a bipolar bubble structure, with an overall span of 1 degree by 3 degrees (140 parsecs × 430 parsecs), extending above and below the Galactic plane and apparently associated with the Galactic Centre. The structure is edge-brightened and bounded, with symmetry implying creation by an energetic event in the Galactic Centre. We estimate the age of the bubbles to be a few million years, with a total energy of 7 × 1052 ergs. We postulate that the progenitor event was a major contributor to the increased cosmic-ray density in the Galactic Centre, and is in turn the principal source of the relativistic particles required to power the synchrotron emission of the radio filaments within and in the vicinity of the bubble cavities