“Fermi” bubbles are bursting from our galaxy. Their origins remain a mystery

[Geee]

PNAS.Org

On a clear night, far from city lights, the glittering plane of the Milky Way splashed across the sky can be an unparalleled sight. But if gamma rays were visible to our eyes, you’d see two massive clouds ballooning up and down from the center of our galaxy, reaching the constellation Virgo in one direction and the constellation Grus in the other. And if X-rays were visible, they’d show the rounded caps crowning those gigantic bubbles, which extend out twice as far.

 

These structures, which seem to have similar origins, are known as the Fermi and eROSITA bubbles. They were only discovered in 2010 and 2020, respectively, and are named for the satellites that spotted them: NASA’s Fermi gamma-ray space telescope and the joint Russian–German eROSITA X-ray telescope (1–3). These colossal formations were a complete surprise; their causes continue to provoke heated debate.

Two competing theories suggest different mechanisms for creating these galaxy-sized bubbles. One posits that they appeared when the supermassive black hole at the center of the Milky Way consumed many thousand suns’ worth of matter and then belched out a bright beam of particles and radiation; the other argues that the culprit was outflowing material driven by an intense period of star formation in our galactic center. There are even hybrid schemes that combine elements of both processes to explain the observed data.

 

“There’s no conclusion or consensus whatsoever about the [bubbles’] origin,” says Hsiang-Yi Karen Yang, an astronomer at the National Tsing Hua University in Hsinchu, Taiwan.

 

New evidence from different experiments could help solve this dilemma. But, regardless of their cause, the Fermi and eROSITA bubbles reveal our galaxy to be a strange and dynamic place. Once thought to be relatively mellow and mature, the Milky Way appears to be experiencing events normally seen only in distant—and therefore much younger—galaxies. Since such processes affect the evolution of galaxies, these odd formations in our cosmic backyard might be our best bet to learn more about such galaxy-shaping energetic events elsewhere in the universe.