Supermassive Black Hole is Blowing Bubbles at the Heart of the Milky Way

Astronomers believe they have discovered the origins of two enormous bubbles being blown out from the heart of our galaxy, tracing them back to the Milky Way's central supermassive black hole.

The study of these bubbles, which extend out around 36,000 light-years above and below the heart of the Milky Way, could lead to a better understanding of how supermassive black holes that sit at the heart of most galaxies grow to tremendous sizes, and how they influence galactic evolution.

The team suggests the bubbles—named the eRosita bubbles after the telescope that found them in 2020—are the result of powerful jet activity launched by the Milky Way's supermassive black hole, Sagittarius A*. The astronomers also found that the jet spewed material for around 100,000 years, beginning around 2.6 million years ago.

University of Michigan astronomer Mateusz Ruszkowski said in a press release from the institution: "Our findings are important in the sense that we need to understand how black holes interact with the galaxies that they are inside because this interaction allows these black holes to grow in a controlled fashion as opposed to growing uncontrollably."

Ruszkowski is the co-author of a paper published in Nature Astronomy that discusses the findings surrounding the formation of these and similar bubbles that sit inside them, first discovered in 2010 and named Fermi bubbles—also after the telescope that found them.

There are two theories of how the structures like the eRosita bubbles and Fermi bubbles, the latter of which are about half the size of the former, could form. One suggestion, the starburst model, involves powerful supernova explosions that occur at the end of massive stars' lifetimes pushing out material.

eRosita Bubbles
A NASA visualization of bubbles extending out from above and below the center of the galaxy and stretching over 36,000 light-years. New research suggests that these eRosita bubbles are being driven by energetic outflows from its supermassive black hole. Gaia/DPAC/NASA/ESA

The other involves outflows and being driven by energy supplied in supermassive black holes. These outflows occur when material approaches a black hole but is not swallowed, and instead moves to the poles of the massive stellar remnant. This results in material being thrown back out into space, preventing the black hole from growing, but also pushing other material it encounters aside like a snowplow.

Ruszkowski said the team's findings support this second model. "If you believe in the model of these Fermi or eRosita bubbles as being driven by supermassive black holes, you can start answering these profound questions," he said.

The astronomers suggest as they expanded the more diminutive Fermi bubbles could have actually pushed out energy or a shockwave that push out the larger eRosita bubbles.

Ruling Out Supernovas

Ellen Zweibel, professor of astronomy and physics at the University of Wisconsin, explained why the findings could rule out the starburst model. She said the typical duration of a nuclear starburst, and therefore the length of time into which a starburst would inject the energy that forms the bubbles, is about 10 million years.

"On the other hand, our active black hole model accurately predicts the relative sizes of the eRosita x-ray bubbles and the Fermi gamma-ray bubbles, provided the energy injection time is about one percent of that or one-tenth of a million years," she said in a statement.

"Injecting energy over 10 million years would produce bubbles with a completely different appearance. It's the opportunity to compare the x-ray and gamma-ray bubbles which provides the crucial previously missing piece."

Studying the eRosita bubbles could also provide astronomers with a wealth of data about jets launched by black holes.

"We not only can rule out the starburst model, but we can also fine-tune the parameters that are needed to produce the same images, or something very similar to what's in the sky, within that supermassive black hole model," Ruszkowski said.

"We can better constrain certain things, such as how much energy was pumped in, what's inside these bubbles and how long was the energy injected in order to produce these bubbles."

The researchers also predict the cosmic rays—high-energy radiation and particles—that are inside the eRosita bubbles and the Fermi bubbles that sit within them like a cosmic nesting doll.

Lead author Karen Yang, who is assistant professor at the National Tsing Hua University in Taiwan, said in a statement: "Our simulation is unique in that it takes into account the interaction between the cosmic rays and gas within the Milky Way. The cosmic rays, injected with the jets of the black hole, expand and form the Fermi bubbles that shine in gamma-rays.

"The same explosion pushes gas away from the Galactic center and forms a shock wave that is observed as the eRosita bubbles. The new observation of the eRosita bubbles has allowed us to more accurately constrain the duration of the black hole activity, and better understand the past history of our own galaxy."