Our Milky Way is considered to be a relatively quiescent galaxy, and yet – at its heart – it’s known to have a 4-million-solar-mass black hole: the source of many fascinating and dynamic processes. Yesterday – September 11, 2019 – astronomers announced the discovery in that region of what they’re calling “one of the largest features ever observed” in the center of the Milky Way. This feature is a pair of enormous radio-emitting bubbles, towering above and below the central region of our galaxy. Scientists described it as hourglass-shaped. The entire structure stretches some 1,400 light-years, or about 5% of the distance between our sun and the galaxy’s center.
… dwarfs all other radio structures in the galactic center [and] is likely the result of a phenomenally energetic burst that erupted near the Milky Way’s supermassive black hole a few million years ago.
In other words, said these scientists, they believe features have formed from a violent eruption, presumably emanating from the vicinity of the galactic center and its supermassive black hole, which – over a short period of time – punched through the interstellar medium in opposite directions. As explained in Nature:
The bubbles are gas structures that can be observed because electrons stirring inside them produce radio waves as they are accelerated by magnetic fields.
The team of astronomers that made the discovery was led by Ian Heywood of the University of Oxford in England. They used the new and supersensitive South African Radio Astronomy Observatory (SARAO) MeerKAT radio telescope to map out broad regions in the center of the galaxy. They conducted their radio observations at wavelengths near 23 centimeters (about 9 inches), which, they said:
… indicates energy 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 our galaxy.
Heywood, who processed the large amount of observational data leading to this result, said:
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 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.
Previously unseen? Yes, in the radio part of the spectrum. But there’s another hourglass-shaped structure previously known by astronomers that might (or might not) be related to the MeerKAT bubbles. And that is the so-called Fermi Bubbles, confirmed by high-energy gamma ray observations in 2010.
I asked one of the authors on this new paper – Fernando Camilo, SARAO Chief Scientist in Cape Town, South Africa – how the new discovery relates to the Fermi Bubbles. He replied by email:
That’s a very good question.
The Fermi bubbles are much larger than the MeerKAT radio bubbles (about 50 times larger: some 75,000 light years in size for Fermi, 1,400 light years for MeerKAT). They are also much more energetic: the amount of energy involved in the event that inflated the MeerKAT bubbles is no more than 1% of the energy content of the Fermi bubbles.
However, they are both huge bi-polar structures, symmetric about the galactic center, near the central supermassive black hole, and so your question does arise.
Our view is that the MeerKAT bubbles may well represent a less energetic version of a process similar to that which created the Fermi Bubbles (the origin of the Fermi bubbles themselves continues to be greatly debated, and I expect that the origin of the MeerKAT bubbles will likewise elicit a range of views).
If that’s the case, the MeerKAT bubbles may well be an example of a series of such intermittent events that occasionally take place near the center of the Milky Way, governed by the black hole, the cumulative effect of which is responsible for other large scale structures seen at higher galactic latitudes (that is, away from the plane of the Milky Way), including structures seen in X-rays and, indeed, the Fermi gamma-ray Bubbles.
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’re witnessing in the Milky Way a novel manifestation of galaxy-scale outflows of matter and energy, ultimately governed by the central black hole.
Bottom line: Radio astronomers have spied a pair of enormous radio-emitting bubbles that tower hundreds of light-years above and below the central region of our galaxy.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.