In 2020, the X-ray satellite eROSITA revealed two gigantic bubbles extending to 80° above and below the center of our Milky Way Galaxy. The morphology of these ‘eROSITA bubbles’ bears a remarkable resemblance to the Fermi bubbles previously discovered by NASA’s Fermi Gamma-ray Space Telescope and its counterpart — the microwave haze, a fog of charged particles roughly at the center of the Galaxy. The physical origin of these striking structures has been intensely debated; however, because of their symmetry about the Galactic center, they probably originate from some energetic outbursts from the Galactic center in the past. In a new paper in the journal Nature Astronomy, astronomers propose a theoretical model in which the eROSITA bubbles, Fermi bubbles and the microwave haze could be simultaneously explained by a single event of jet activity from Sagittarius A* — the 4.3-million-solar-mass black hole at Milky Way’s center — that occurred about 2.6 million years ago and lasted about 100,000 years.
A visualization of the eRosita and Fermi bubbles. Image credit: ESA / Gaia / DPAC / CC BY-SA 3.0 IGO.
“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,” said Dr. Mateusz Ruszkowski, an astronomer in the Department of Astronomy at 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.”
There are two competing models that explain these bubbles, called Fermi and eRosita bubbles after the telescopes that named them.
The first suggests that the outflow is driven by a nuclear starburst, in which a star explodes in a supernova and expels material.
The second model, supported by the new findings, suggests that these outflows are driven by energy thrown out from a supermassive black hole at the center of our Galaxy.
These outflows from black holes occur when material travels toward the black hole, but never crosses the black hole’s event horizon, or the mathematical surface below which nothing can escape.
Because some of this material is thrown back into space, black holes don’t grow uncontrollably.
But the energy thrown from the black hole does displace material near the black hole, creating these large bubbles, which are nearly 36,000 light-years tall.
The eRosita bubbles are about two times the size of the Fermi bubbles and are expanded by the wave of energy, or a shockwave, pushed out by the Fermi bubbles.
“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,” Dr. 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.”
To arrive at their conclusions, Dr. Ruszkowski and colleagues performed numerical simulations of energy release that take into account hydrodynamics, gravity and cosmic rays.
“Our simulation is unique in that it takes into account the interaction between the cosmic rays and gas within the Milky Way,” said Dr. H.-Y. Karen Yang, an astronomer in the Institute of Astronomy and the Department of Physics at Taiwan’s National Tsing Hua University and the Physics Division at Taiwan’s National Center for Theoretical Sciences.
“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.”
“Our model rules out the nuclear starburst theory because 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,” said Professor Ellen Zweibel, an astronomer in the Department of Astronomy at the University of Michigan.
“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.”
“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.”
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HY.K. Yang et al. Fermi and eROSITA bubbles as relics of the past activity of the Galaxy’s central black hole. Nat Astron, published online March 7, 2022; doi: 10.1038/s41550-022-01618-x
