First images of giant radio-emitting bubbles to throw more light on supermassive black hole at the center of the Milky Way

These features, which were captured by the MeerKAT telescope, were formed from a violent eruption near the supermassive black hole several million years ago, say scientists


                            First images of giant radio-emitting bubbles to throw more light on supermassive black hole at the center of the Milky Way

An hourglass-like feature, which dwarfs all other structures at the center of the Milky Way, has been discovered by an international team of astronomers. The newly spotted pair of radio-emitting bubbles reach hundreds of light-years tall. According to the researchers, the towering balloon-like structure is the result of a "phenomenally energetic burst" that erupted near the Milky Way's supermassive black hole several million years ago.

"The center of our galaxy is relatively calm when compared to other galaxies with very active central black holes. 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 is possible that one such feeding frenzy triggered powerful outbursts that inflated this previously unseen feature," says the lead author of the study Ian Heywood from the University of Oxford.

The research team - from across 15 institutions - used the South African Radio Astronomy Observatory (SARAO) MeerKAT telescope (as seen in the image above by SARAO/Oxford/NRAO), the largest science project in Africa. This is the first paper detailing research completed with MeerKAT's full 64-dish array since its launch on July 2018. The findings have been published in the journal Nature

"The shape and symmetry of what we have observed strongly suggest that a staggeringly powerful event happened a few million years ago very near our galaxy's central black hole. 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," says study co-author William Cotton, from the National Radio Astronomy Observatory in Charlottesville, Virginia. 

Cotton adds, "In effect, this inflated energetic bubbles in the hot, ionized gas near the galactic center, energizing it and generating radio waves which we could eventually detect here on Earth."

Mysteries of the Galaxy uncovered

More turbulent and unusually active as compared to the rest of the Milky Way, the environment surrounding the galaxy's central black hole holds many mysteries. According to the research team, MeerKAT has unprecedented sensitivity and imaging capabilities which, coupled with its geographic vantage point for observing the Galactic center, has resulted in the clearest ever image of the radio waves emanating from the center of the Milky Way, a part of space that is notoriously difficult to image at such wavelengths.

Using the MeerKAT telescope, the researchers mapped out broad regions at 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 free-floating electrons are accelerated as they interact with powerful magnetic fields. This produces a characteristic radio signal that can be used to trace energetic regions in space. The radio light seen by MeerKAT penetrates the dense clouds of dust that block visible light from the center of the galaxy," the findings state.

A radio image of the central portions of the Milky Way galaxy. The plane of the galaxy is marked by a series of bright features, exploded stars and regions where new stars are being born, and runs horizontally through the image. The radio bubbles discovered by MeerKAT extend vertically above and below the plane of the galaxy. (SARAO/Oxford)

By analyzing the nearly identical "extent and morphology" of the twin bubbles, the scientists believe 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."

"It is extremely exciting to be able to peer at the center of the Galaxy with such high definition and precision. This is the nearest supermassive black hole to us in the Universe, and MeerKAT has provided us with front row center seats. We are going to learn an enormous amount about how black holes feed themselves and how they influence their environments," says professor Steve Balbus, head of astrophysics at the University of Oxford. 

The research team says that the new observational capabilities are unlocking a "fossil record," which allows scientists to piece together the history of the Galactic center and the supermassive black hole that lurks there. The scientists say that even though the structure is likely to be a few million years old, it is still possible to observe it, and from there, infer from where it came.

"These enormous bubbles have until now been hidden by the glare of extremely bright radio emission from the center of the galaxy. Teasing out the bubbles from the background 'noise' was a technical tour de force, only made possible by MeerKAT's unique characteristics and propitious location in the Southern hemisphere. With this unexpected discovery, we are witnessing in the Milky Way a novel manifestation of galaxy-scale outflows of matter and energy, ultimately governed by the central black hole," says study co-author Fernando Camilo of SARAO in Cape Town.

The event could also be the origin of the population of electrons that are required to power the radio emission from mysterious magnetized filaments, say astronomers. In the early 1980s, largescale, and highly organized magnetic filaments were discovered in the center of the Milky Way, 25,000 light-years from Earth. Their origin has remained an unsolved mystery despite the filaments being radio wave-emitting structures that stretch tens of light-years long and one light-year wide

"The radio bubbles discovered with MeerKAT now shed light on the origin of the filaments. Almost all of the more than 100 filaments are confined by the radio bubbles," says co-author Farhad Yusef-Zadeh, a professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). 

The researchers believe 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.

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