Strong outflow winds at the center of the galaxy may cause “Fermi bubbles”

The rapid air flow from the galactic center creates both positive and reverse shocks. The latter forms the outline of the Fermi bubble. Image source: Tokyo Metropolitan University

Research by scientists at Tokyo Metropolitan University has proved that large gamma-emitting bubbles around the center of the Milky Way are created by rapidly blowing outward winds and associated “reverse shocks.” Numerical simulations successfully reproduced the temperature distribution observed by X-ray telescopes. Such jets have also been observed in other galaxies; The discovery suggests that until recently, similar winds may have been blowing through the galaxy. The research is published in the latest issue of the Monthly Bulletin of the Royal Astronomical Society in 2023.

The universe is full of massive objects that have yet to be explained by scientists. One of them is the “Fermi bubbles,” so named because they were first discovered in 2010 by the Fermi Gamma-ray Space Telescope. These bubbles are huge gamma-ray emission regions that stretch about 50,000 light-years on either side of the galactic center, protruding from the galactic plane like balloons (pictured). Despite its staggering scale, the mechanisms by which it was formed have yet to be deciphered.

Now, Tokyo Metropolitan University professor Yutaka Fujita has presented theoretical evidence to show how these celestial bodies formed. Since the discovery of the Fermi bubble, scientists have proposed many hypotheses about its formation, including explosive activity from the central supermassive black hole, wind from the black hole, and stable star-forming activity. Distinguishing between these situations is a challenging task, but the Suzaku satellite’s state-of-the-art X-ray observations provide an opportunity for scientists to compare measurements with expectations from different scenarios.

Fujita’s simulations suggest that a rapid outflow of wind from a black hole injects the necessary energy into the gas around the center of the galaxy. Compared to the measured profile, they found that the Fermi bubbles were most likely created by fast-flowing winds that blew by at 1,000 kilometers per second over 10 million years. Unlike winds on Earth, these winds move at high speeds and are streams of highly charged particles that travel through space. These winds move outward and interact with the surrounding “halo”, causing a “reverse shock” that creates a characteristic temperature peak. Fermi bubbles correspond to the volume inside the front of the reverse shock. Importantly, the simulations also showed that the instantaneous explosion at the center could not reproduce the contours measured by the telescope, which provided the basis for scenarios based on the steady winds generated by the central black hole.

The researchers note that the wind predicted by the simulations is similar to the jets observed in other galaxies. This correspondence suggests that the massive outflows seen elsewhere in the universe have only recently appeared in the Milky Way. (Source: Feng Weiwei, China Science News)

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