The next story comes to us from a collaboration that includes one of the most prolific researchers in astronomy: Avi Loeb. Working in collaboration with Giacomo Fragione, this pair looked at the motions of the stars orbiting closest to the supermassive black hole in the center of the Milky Way to see if they could use the stellar motions to get at the rotation rate of the black hole. 

If you have a fast-rotating black hole (one going up to as fast as the speed of light), the twisting of space-time will affect the motions of objects around it, effectively scattering them more and more over time. This is like the effects of a hurricane on the motions of things trying to move near it; they simply get carried in new directions. If the black hole, however, isn’t rotating quickly, there is no scattering effect.

In looking at the S-stars orbiting the Milky Way, astronomers find two distinct populations orbiting in different planes. It’s believed these are stars that formed from two distinct star-forming regions and have since essentially migrated into rings of stars, as those a bit closer to the center of the galaxy orbited faster over the eons, and those a little further away orbited a bit slower.

Since both these rings of stars are still well defined, Fragione and Loeb calculate that our supermassive black hole, Sag A*, must be rotating no faster than 10% the speed of light.

This speed has consequences for how our galaxy can behave. With this low speed, infalling material won’t be whipped up into a quickly turning accretion disk that drives powerful magnetic fields that accelerate jets of material out of our galaxy. In fact, our system is likely to remain jet free through future interactions.

According to Fragione, this will also have an effect on future observations by the Event Horizon Telescope. Here, I’d like to admit, this quote is how I learned there are planned additional observing runs with the EHT. I can’t wait to see what the processed images eventually show, and I look forward to seeing this result of a slow rotating supermassive black hole confirmed. 

More Information:

Center for Astrophysics press release 

“An Upper Limit on the Spin of Sgr A* Based on Stellar Orbits in Its Vicinity,” Giacomo Fragione & Abraham Loeb, 2020 Oct. 1, Astrophysical Journal Letters (preprint on arxiv.org)