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Towering balloon-like features discovered near center of the Milky Way

September 11, 2019

An international team of astronomers has discovered one of the largest features ever observed in the center of the Milky Way - a pair of enormous radio-emitting bubbles that tower hundreds of light-years above and below the central region of our galaxy.

This hourglass-like feature, which dwarfs all other radio structures in the galactic center, is likely the result of a phenomenally energetic burst that erupted near the Milky Way's supermassive black hole a few million years ago.

"The center of our galaxy is relatively calm when compared to other galaxies with very active central black holes," said Ian Heywood of the University of Oxford and lead author of an article appearing in the journal Nature. "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."

Using the South African Radio Astronomy Observatory (SARAO) MeerKAT telescope, Heywood and his colleagues mapped out broad regions in the center of the galaxy, conducting observations at wavelengths near 23 centimeters. Radio emission of this kind is generated in a process known as synchrotron radiation, in which electrons moving at close to the speed of light interact with powerful magnetic fields. This produces a characteristic radio signal that can be used to trace energetic regions in space. This radio light easily penetrates the dense clouds of dust that block visible light from the center of the galaxy.

By examining the nearly identical size and shape of the twin bubbles, the researchers think they have found convincing evidence that these features were formed from a violent eruption that over a short period of time punched through the interstellar medium in opposite directions.

"The shape and symmetry of what we have observed strongly suggests that a staggeringly powerful event happened a few million years ago very near our galaxy's central black hole," said William Cotton, an astronomer with the National Radio Astronomy Observatory in Charlottesville, Virginia, and co-author on the paper. "This eruption was possibly triggered by vast amounts of interstellar gas falling in on the black hole, or a massive burst of star formation which sent shockwaves careening through the galactic center. In effect, this inflated bubbles in the hot, ionized gas near the galactic center, energizing it and generating radio waves that we could eventually detect here on Earth."

The environment surrounding the black hole at the center of our galaxy is vastly different than the environment elsewhere in the Milky Way, and is a region of many mysteries. Among those are very long and narrow filaments found nowhere else, the origin of which has remained an unsolved puzzle since their discovery 35 years ago. The filaments appear as radio structures tens of light-years long and approximately a light-year wide.

"The radio bubbles discovered by MeerKAT now shed light on the origin of the filaments," said Farhad Yusef-Zadeh at Northwestern University in Evanston, Illinois, and a co-author on the paper. "Almost all of the more than one hundred filaments are confined by the radio bubbles."

The authors suggest that the close association of the filaments with the bubbles implies that the energetic event that created the radio bubbles is also responsible for accelerating the electrons required to produce the radio emission from the magnetized filaments.

"These enormous bubbles have until now been hidden by the glare of extremely bright radio emission from the center of the galaxy," said Fernando Camilo of SARAO in Cape Town, and co-author on the paper. "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," according to Camilo. "With this unexpected 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."

According to the researchers, the discovery of these bubbles relatively nearby in the center of our home galaxy brings us one step closer to understanding spectacular activities that occur in more distant cousins of the Milky Way throughout the universe.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Reference: "Inflation of 430-parsec bipolar radio bubbles in the Galactic Centre by an energetic event," I. Heywood, et al., Nature.

National Radio Astronomy Observatory

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