3 Jan. () –
The large gamma-ray emitting bubbles around the center of the Milky Way were produced by fast outward blowing winds and an associated “reverse shock”.
This has been demonstrated by a scientist from Tokyo Metropolitan University in numerical simulations that successfully reproduced the temperature profile observed by an X-ray telescope. The finding is published in Monthly Notices of the Royal Astronomical Society. These flows have been observed in other galaxies; this finding suggests that similar winds they could have blown into our galaxy until very recently.
The Universe is full of massive celestial objects that have not yet been explained. Among them are the “Fermi bubbles”, so called because they were first discovered by the Fermi gamma-ray space telescope in 2010. These bubbles are huge gamma-ray emitting regions that extend from either side of the center of the Milky Way for approximately 50,000 light-years, protruding from the plane of the galaxy like balloons, as shown in the figure above. Despite their mind-boggling scale, the mechanism by which they form has yet to be deciphered.
Now, Professor Yutaka Fujita has presented theoretical evidence demonstrating how these objects may have formed. Since their discovery, many hypotheses have been put forward about the formation of Fermi bubbles, including explosive activity from the central supermassive black hole, winds from the black hole, and ongoing star formation activity. Distinguishing these scenarios is a difficult task, but the availability of state-of-the-art X-ray observations from the Suzaku satellite offers an opportunity to compare the measurements with what astronomers expect from the various scenarios.
Professor Fujita’s simulations considered fast outflow winds from the black hole that inject the necessary energy into the gas surrounding the center of the galaxy. By comparing them to the measured profiles, they found that the Fermi bubbles were most likely produced by fast outflow winds, which blow at 1,000 km per second for 10 million years. These are not winds like we experience on Earth, but of streams of highly charged particles that travel at high speed and propagate through space.
These winds travel outward and interact with the surrounding halo gas, causing a reverse shock that creates a characteristic temperature spike. The Fermi bubbles correspond to the volume located inside this inverse shock front. Importantly, the simulations also showed that an instantaneous explosion at the center could not reproduce the profiles measured by the telescope, which gives weight to a scenario based on constant winds generated by the central black hole.
The author points out that the winds predicted by the simulation are similar to the outflows observed in other galaxies. The correspondence suggests that the same types of mass flows observed in other parts of the universe were present in our galaxy until recently.
