The Milky Way's black hole burped out two X-ray bubbles

Our galaxy has blown some bubbles. Astronomers have spotted a pair of enormous bubbles of plasma extending above and below the Milky Way and emitting X-rays, and they probably came from an extraordinary event in the galaxy’s centre.

Andrea Merloni at the Max Planck Institute for Extraterrestrial Physics in Germany and his colleagues found these balloons of hot plasma using the eROSITA X-ray telescope aboard the Spektr-RG space observatory. Pictures of the entire sky from eROSITA revealed giant spheres of gas extending more than 45,000 light years above and below the disc of the galaxy. For comparison, the Milky Way is about 105,000 light years across.

Astronomers had already spotted the bubble above the galaxy, but we didn’t have any X-ray telescopes sensitive enough to see the more tenuous southern one. “Because it was only one-sided, people had a lot of trouble figuring out what they were and where they come from,” says Merloni. “Now we finally see the southern bubble, so a lot of the controversy about the northern bubble is resolved.”

Before, we couldn’t be sure whether the northern bubble came from the centre of the galaxy or if it was actually a closer structure that we just happened to be viewing from an angle that gave us that impression. With these new observations, it is almost certain that the former is true, but we still don’t know why the southern bubble is less dense than the northern one, says Merloni.

These X-ray bubbles were probably caused by the same event as the Fermi bubbles, which are similar but slightly smaller structures that emit gamma rays. The enormous size of the eROSITA bubbles gives us some insight into what kind of event could have caused both sets of spheres. It could have been a shock wave from a powerful episode of stellar birth near the centre of the galaxy, but it is tough to get enough energy from star formation to make such a powerful shock wave.

Instead, the bubbles were probably formed when matter fell into our galaxy’s central supermassive black hole, releasing an extraordinary blast of energy that ploughed through the gassy halo around the galaxy. “It would be no problem to have a little bit of gas falling onto the black hole and releasing the energy required to inflate these bubbles,” says Merloni. The eROSITA mission is expected to last at least six more years, so we should be able to learn more from its later observations.

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