The Milky Way’s black hole is spinning rapidly

Sagittarius A*

February 17, 2024 9:34 p.m. Robert Klatt

The rotation of Sagittarius A* reaches 60% of the theoretical maximum speed defined by Einstein. The currently inactive black hole at the center of the Milky Way could generate powerful jets as soon as it devours matter again.

Reading (United States). Sagittarius A*, the black hole at the center of the Milky Way, influences the entire galaxy with its gravity. The black hole, which weighs around four million solar masses, is currently relatively inactive. Astronomy cannot therefore determine exactly whether and how quickly the black hole rotates. However, the dynamic mass of Sagittarius A* and nearby spacetime are influenced by spin.

Astronomers have therefore attempted to determine the rotation speed using various methods, including model calculations and the motion of nearby stars. They arrived at very different results, according to which the black hole moves from very slowly to a speed close to the theoretical maximum speed defined by Einstein.

Rotation speed of Sagittarius A*

Researchers led by Ruth Daly of Pennsylvania State University (PSU) have, according to a publication in Monthly Notices of the Royal Astronomical Society Let us now determine more precisely the rotation speed of Sagittarius A*. They combined X-ray and radio data measured in parallel to determine what materials and radiation the black hole produces given its current low activity. The measurement data also made it possible to observe the behavior of matter at the event horizon.

“Our data suggests that Sagittarius A* rotates very quickly.”

60 percent of theoretical maximum speed

According to measurement data, the angular velocity of Sagittarius A* is 60% of the theoretical maximum velocity. This means the black hole can produce powerful jets as it devours matter again.

“About 15 percent of its dynamic mass can then be extracted.”

As Megan Donahue explains, this can explain various observations in the galaxy, including sudden star formation about a billion years ago and Fermi bubbles of gamma rays and fast particles.

“Jets produced by a rotating black hole can have a lasting impact on the gas supply of an entire galaxy, which in turn influences star formation and number.”

According to Anan Lu, the information on the rotation of the black hole also shows how the black hole will behave in the future. Once it devours nearby stars or gas again, it will become active again and could form jets of radiation and fast-moving particles that extend far into space. However, it is impossible to predict when this will happen.

“It could happen in a thousand years, or in a million years, but also in our lifetime.”

Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnras/stad3228