Sagittarius A* – the supermassive black hole at the Milky Way's core – is believed to have created a series of enormous bubbles stretching for tens of thousands of light-years above and below the Milky Way's plane. A new study has proposed a model to explain how that might have happened

Over a decade ago, astronomers using NASA’s Fermi Gamma-Ray Space Telescope discovered the so-called Fermi bubbles, vast structures of extremely hot gas stretching 25,000 light-years. More recently, the eROSITA observatory detected the eROSITA bubbles stretching even further to almost 36,000 light-years.

Their origin is uncertain, but the fact that they are symmetric with respect to the galactic disk means there is a likely culprit: Sagittarius A*.

As reported in Nature Astronomy, the new model connects the Fermi Bubbles, the eROSITA bubbles, and the microwave haze surrounding the galactic center. In that scenario, all these components are the consequence of a single event of jet activity from the supermassive black hole that happened a few million years ago.

A computer model took into account how gas flows around the Milky Way (especially its core), the energy that jets produced by a black hole can deliver, the gravity of the system, and the presence of high-energy particles known as cosmic rays.

“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,” lead author Professor Karen Yang, from the National Tsing Hua University in Taiwan, said in a statement.

“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.”

Their work finds no support for an alternative view that saw an increased formation of stars (a so-called starburst) in the Milky Way's core as the cause of the bubbles. It would need such an event to last 10 million years, and such a prolonged injection of energy would make, according to the simulation, dramatically different structures. The black hole jet scenario instead needs to last just 100,000 years.

“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 grow uncontrollably,” added co-author Mateusz Ruszkowski, from the University of Michigan. “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.”