The giant black holes in the centers of galaxies are spinning faster than ever before on average, two U.K. astronomers have concluded based on telescope observations.
Alejo Martinez-Sansigre of the University of Portsmouth and Steve Rawlings of the University of Oxford are reporting the findings in the journal Monthly Notices of the Royal Astronomical Society.
Scientists believe most galaxies harbor at their centers huge, “supermassive” black holes that weigh the equivalent of a million to a billion times our sun. Black holes are objects so dense and heavy that their gravity sucks in anything that strays too close, even light rays. While black holes can’t be seen directly, it’s possible to see the material that’s gradually falling into a black hole. Usually spiraling inward and forming a disk shape around the central mass, the material can become very hot and emit radiation including X-rays detectable by space-based telescopes. Radio waves are also emitted, detectable from ground telescopes.
Twin jets of particles often spray out and away from black holes and their “accretion disks.” How these jets arise is unclear, but a major factor is thought to be that black holes are spinning. Physicists have thus been trying to learn more information about black hole spins, including how these might be changing.
The spin of black holes can also “tell you a lot about how they formed,” Martinez-Sansigre said. “Our results suggest that in recent times a large fraction of the most massive black holes have somehow spun up. A likely explanation is that they have merged with other black holes of similar mass, which is a truly spectacular event, and the end product of this merger is a faster-spinning black hole.”
Martinez-Sansigre and Rawlings compared theoretical models of spinning black holes with telescope observations using radio, X-ray and visible-light data. They concluded that existing theories can explain the population of supermassive black holes with jets.
Using radio observations, the astronomers sampled the population of black holes, deducing the spread of the power of the jets. By estimating how fast they draw in material, the researchers could then infer how quickly these objects are spinning. The spins could then be compared at different times in the universe, because astronomers can get a look at earlier periods simply by looking further away. This works because the light from more distant objects takes longer to reach us, revealing them as they appeared in a more distant past.
“Later this decade we hope to test our idea that these supermassive black holes have been set spinning relatively recently,” Rawlings said. “Black hole mergers cause predictable distortions in space and time—so-called gravitational waves. With so many collisions, we expect there to be a cosmic background of gravitational waves.” This in turn should “change the timing of the pulses of radio waves that we detect from the remnants of massive stars known as pulsars,” he added. “If we are right, this timing change should be picked up by the Square Kilometre Array, the giant radio observatory due to start operating in 2019.”
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