Astronomers have spotted two giant ‘bubbles’ of radio energy around the Milky Way’s central black hole. They hint at a violent past for the galaxy’s heart.

A super-sensitive South African telescope has detected two giant radio bubbles emanating out of the galactic center.

The South African Radio Astronomy Observatory’s MeerKAT radio telescope is an array of 64 dishes spread out over a region eight kilometers across near Carnarvon. The completed telescope began taking data in spring 2018.

So, the bubbles stretch over a total of 430 parsecs (1,400 light-years). That’s about 5% of the distance between the Solar System and the Galaxy’s center. They are a sign of an ancient energetic explosion.

Astronomers discovered these gargantuan structures in 2010 while looking toward the center of the galaxy with NASA’s Fermi Gamma-ray Space Telescope.

No one really knows when this galactic bubble-blowing blast occurred. However, researchers reported some fresh clues found by looking to the opposite end of the electromagnetic spectrum, at radio waves.

The bubbles are gas structures that appear only in X-ray and gamma-ray light. As magnetic fields accelerate them, they produce radio waves. This activity suggests the bubbles are the remnants of an energetic eruption of hot gas several million years ago.

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One possibility is that a few million years ago the central supermassive black hole underwent a period of intense matter-gobbling that created the outburst, the researchers suggest. Another could be that an event involving vast amounts of energy — equivalent to the explosion of about 100 stars — sent matter spewing out of the region around the black hole a few million years ago. The shock waves produced in that event could have created the bubbles’ radio waves.

“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,” study co-author Ian Heywood,  an astrophysicist at the University of Oxford in the United Kingdom, said in a statement. “It’s possible that one such feeding frenzy triggered powerful outbursts that inflated this previously unseen feature.”

Based on the speed of gas flowing near the bottom of the radio bubbles, the researchers estimated the structures to be about 7 million years old. That aligns with younger estimates for the ages of the Fermi Bubbles. Therefore, it’s possible that the two sets of bubbles resulted from the very same cosmic eruption — or, at least, the same sort of explosion.

The bubbles could solve an old puzzle in radio astronomy. The electrons accelerating inside them could be the source of bright ‘filaments’ of matter tens of parsecs long that stretch out of the Galactic center, first seen in 1984.

Researchers published a paper about the study today in Astrophysical Journal Letters.

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