H1821+643, Credit: NASA
NASA’s Chandra X-ray Observatory has enabled an unprecedented measurement of a black hole’s spin, making it slower than most of its smaller cousins.
This is the most massive black hole with an accurate spin measurement and it gives clues as to how some of the largest black holes in the universe grow.
Supermassive black holes contain millions or even billions of times more mass than the Sun. Astronomers believe that almost all large galaxies have a supermassive black hole at their center. While the existence of supermassive black holes is not in dispute, scientists are still working to understand how they grow and evolve. One critical piece of information is the speed at which black holes are spinning.
“Each black hole can be defined by just two numbers: its spin and its mass,” Julia Sisk-Reynes of the Institute for Astronomy (IoA) at the University of Cambridge, who led the new study, said in a statement. “While that sounds simple enough, calculating those values for most black holes has turned out to be incredibly difficult.”
For this result, the researchers looked at X-rays bouncing off a disk of material spinning around the black hole in a quasar known as H1821+643. Quasars contain fast-growing supermassive black holes that generate large amounts of radiation in a small region around the black hole.
Located in a cluster of galaxies about 3.4 billion light-years from Earth, the black hole in H1821+643 has between 3 billion and 30 billion solar masses, making it one of the most massive known. By contrast, the supermassive black hole at the center of our galaxy weighs about four million suns.
“We found that the black hole in H1821+643 rotates about half as fast as most black holes weighing between a million and ten million Suns,” said co-author Christopher Reynolds, also of the IoA. “The million dollar question is: why?”
The answer may lie in how these supermassive black holes grow and evolve. This relatively slow spin supports the idea that the most massive black holes, like H1821+643, experience most of their growth by merging with other black holes, or because gas is pulled inward in random directions when their large disks break apart. .
Supermassive black holes that grow in this way are likely to often undergo large spin changes, including slowing down or twisting in the opposite direction. Therefore, the prediction is that more massive black holes should be observed to have a wider range of spin rates than their less massive relatives.
On the other hand, scientists expect less massive black holes to accumulate most of their mass from a disk of gas that rotates around them. Because such disks are expected to be stable, incoming matter always approaches from a direction that will cause the black holes to spin faster until they reach the maximum possible speed, which is the speed of light.
“The moderate spin of this ultramassive object may be a testament to the violent and chaotic history of the largest black holes in the universe,” said co-author James Matthews, also of IoA. “It may also give insight into what will happen to our galaxy’s supermassive black hole billions of years in the future, when the Milky Way collides with Andromeda and other galaxies.”
This black hole provides information that complements what astronomers have learned about the supermassive black holes seen in our galaxy and in M87, which were imaged with the Event Horizon Telescope. In those cases, the masses of the black hole are well known, but the spin is not.
A paper describing these results from Sisk-Reynes and her collaborators appears in the Monthly Notices of the Royal Astronomical Society and is available at https://arxiv.org/abs/2205.12974