NASA’s visualization team created an overlay of an image of the Milky Way, taken by the European Space Agency’s Gaia Space Observatory, and a visualization of simulations of the eRosite and Fermi bubbles prepared by Karen Yang (lead author of the study). and assistant professor at National Tsing Hua University in Taiwan) in cooperation with paper co-authors Mateusz Ruszkowski (University of Michigan) and Ellen Zweibel (University of Wisconsin). Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 OIG
In 2020, the eRosita X-ray telescope captured images of two huge bubbles stretching far above and below the center of our galaxy.
Since then, astronomers have debated its origin. Now a study including research from the University of Michigan suggests the bubbles are the result of a powerful jet of activity from the supermassive black hole at the center of the Milky Way. The study, published in Natural Astronomy, also shows that the jet began spewing material about 2.6 million years ago and lasted about 100,000 years.
The team’s results suggest that the Fermi bubbles, discovered in 2010, and the microwave haze, a fog of charged particles roughly in the center of the galaxy, were formed by the same jet of energy from the supermassive black hole. The study was conducted by National Tsing Hua University in collaboration with UM and the University of Wisconsin.
“Our findings are important in that we need to understand how black holes interact with the galaxies they are in, because this interaction allows these black holes to grow in a controlled way rather than [growing] out of control,” he says. UM astronomer Mateusz Ruszkowski, a co-author of the study. “If you think the pattern of these Fermi or eRosita bubbles is powered by supermassive black holes, you can start to answer those deep questions.”
There are two competing models that explain these bubbles, called Fermi and eRosita bubbles after the telescopes that named them, Ruszkowski says. The first suggests that the outflow is driven by a nuclear star explosion, in which a star explodes as a supernova and expels material. The second model, supported by the team’s findings, suggests that these outflows are driven by energy pouring out of a supermassive black hole at the center of our galaxy.
These black hole exits occur when matter moves toward the black hole, but never crosses the black hole’s event horizon or mathematical surface below which nothing can escape. Because some of this material is sent back into space, black holes don’t grow out of control. But the energy emitted by the black hole moves matter near the black hole, creating these big bubbles.
The structures themselves are 11 kiloparsecs tall. A parsec is 3.26 light-years, or about three times the distance light travels in one year. The structures then measure almost 36,000 light years.
For comparison, the Milky Way galaxy is 30 kiloparsecs in diameter and our solar system is about 8 kiloparsecs from the center of the galaxy. The eRosite bubbles are about twice the size of the Fermi bubbles and are expanded by the energy wave, or shock wave, ejected by the Fermi bubbles, the researchers said.
Astronomers are interested in looking at these particular eRosite bubbles because they occur in our own galactic backyard rather than objects in a different galaxy or at extreme cosmological distance. Our proximity to outflows means astronomers can collect a large amount of data, says Ruszkowski. These data can tell astronomers how much energy is in the black hole’s jet, how long that energy was injected, and what material comprises the bubbles.
“Not only can we rule out the starburst model, but we can also adjust the parameters needed to produce the same images, or something very similar to what is in the sky, in this supermassive black hole model,” Ruszkowski said. “We can better constrain certain things, like how much energy is pumped out, what’s inside those bubbles, and how long the energy was injected to produce those bubbles.”
What is inside them? Cosmic rays, a form of high-energy radiation. The eRosita bubbles contain the Fermi bubbles, the contents of which are unknown. But the researchers’ models can predict the number of cosmic rays within each of the structures. The injection of energy from the black hole inflated the bubbles, and the energy itself was in the form of kinetic, thermal, and cosmic ray energy. Among these forms of energy, the Fermi mission was only able to detect the gamma signal from cosmic rays.
Karen Yang, lead author of the study and an assistant professor at National Tsing Hua University in Taiwan, began working on an early version of the code used to model this paper as a postdoctoral researcher at UM with Ruszkowski. To reach their conclusions, the researchers ran numerical simulations of energy release that take into account hydrodynamics, gravity, and cosmic rays.
“Our simulation is unique because it takes into account the interaction between cosmic rays and gas within the Milky Way. Cosmic rays injected with the jets from the black hole expand and form Fermi bubbles that glow in gamma rays,” says Yang.
“The same explosion pushes the gas away from the galactic center and forms a shock wave that is observed when eRosite bubbles. The new observation of eRosite bubbles has allowed us to more precisely constrain the duration of black hole activity and better understand the past history of our own galaxy.”
The researchers’ model rules out the nuclear outburst theory because the typical duration of a nuclear outburst, and thus the duration over which a starburst would inject the energy that forms the bubbles, is about 10 million years, according to the study. co-author of the study. Ellen Zweibel, professor of astronomy and physics at the University of Wisconsin.
“On the other hand, our active black hole model accurately predicts the relative sizes of eRosite X-ray bubbles and Fermi gamma-ray bubbles, as long as the energy injection time is about 1%, or a tenth of a million years. Zweibel said.
“Injecting energy for 10 million years would produce completely different looking bubbles. It is the opportunity to compare X-ray and gamma ray bubbles that provides the crucial piece that was missing before.”
H.-Y. Karen Yang et al, Fermi Bubbles and eROSITA as Relics of Past Activity from the Galaxy’s Central Black Hole, Natural Astronomy (2022). DOI: 10.1038/s41550-022-01618-x
University of Michigan