Large Radio Bubbles Discovered Around The Center Of The Milky Way

A radio image of the centre of the Milky Way with a portion of the MeerKAT telescope array in the foreground. Adapted from results published in Heywood et al. 2019.

Sagittarius A* is the supermassive black hole at the center of the Milky Way. Compared to many supermassive black holes in other galaxies, it is remarkably quiet, but this wasn’t always the case. An international collaboration of researchers has found evidence of intense activity in the last few million years.

As reported in the journal Nature, scientists used the MeerKAT telescope to spot two balloon-like structures towering hundreds of light-years above and below the core of the galaxy. The two bubbles are too similar to be unrelated and the team found an energy signature that can only be explained by some powerful process.

“The centre of our galaxy is calm when compared to other galaxies with very active central black holes,” lead author Ian Heywood of the University of Oxford, 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. It’s possible that one such feeding frenzy triggered powerful outbursts that inflated this previously unseen feature.”

The team looked at wavelengths around 23 centimeters (9 inches), a radio emission associated with synchrotron radiation. When electrons are moving close to the speed of light within powerful magnetic fields, they end up in helical orbit and emit radio waves.

A radio image of the central portions of the Milky Way galaxy. The radio bubbles discovered by MeerKAT extend vertically above and below the plane of the galaxy. Many magnetized filaments can be seen running parallel to the bubbles. Adapted from results published in Heywood et al. 2019.

The discovery also sheds new light on the radio filaments seen around the center of the Milky Way. These are narrow structures seen at the core and nowhere else in our galaxy. Most of them are contained completely within the volume of the bubbles, and it is very likely that whatever event created the bubbles also accelerated the electrons in the filaments.

“These enormous bubbles have until now been hidden by the glare of extremely bright radio emission from the centre of the galaxy,” co-author Fernando Camilo, from the South African Radio Astronomy Observatory in Cape Town, explained. “Teasing out the bubbles from the background noise was a technical tour de force, only made possible by MeerKAT’s unique characteristics and ideal location. With this discovery, we’re witnessing in the Milky Way a novel manifestation of galaxy-scale outflows of matter and energy, ultimately governed by the central black hole.”

From end to end, the bubbles stretch about 1,400 light-years. While these bubbles are impressive, they are not the only bubbles present around the Milky Way. The so-called Fermi bubbles stretch for 25,000 light-years, created by a powerful eruption of material from Sagittarius A*. These new bubbles might have been created by a less energetic version of the same event.


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