Scientists have measured the recoil velocity from a cataclysmic collision between two black holes for the very first time.
Gravitational waves are ripples in space-time first proposed to exist by Albert Einstein, and detected for the first time in 2015. Another first came in 2019, when scientists picked up a gravitational wave signal resulting from a violent merger between vastly different sized black holes. The size imbalance caused the newborn black hole to ricochet off into the universe in a phenomenon known as a “natal kick.”
Now, astronomers have deciphered this gravitational wave signal, called GW190412, revealing that the collision caused the newly-merged black hole to shoot through space at more than 31 miles per second (50 kilometers per second) — fast enough to catapult it out of its original cluster of stars, researchers reported in the study, published on Sept. 9 in the journal Nature Astronomy.
“It’s a remarkable demonstration of what gravitational waves can do,” study co-author Koustav Chandra, an astrophysicist at Pennsylvania State University said in a statement.
Collision signals
When black holes careen toward one another they produce gravitational waves. But when one black hole is much more massive than the other, the gravitational waves produced look very different depending on the angle from which they are observed.
By looking from different angles, researchers can find the direction of the kick. Then, the kick’s speed can be determined by measuring the mass ratio and spin of the two original black holes — information that can also be determined from studying gravitational waves.
If the recoil from the collision is strong enough to slingshot the merged black hole from its star cluster, this prevents this new black hole from subsequently merging with other black holes and potentially forming a supermassive black hole — which can be 100,000 to 50 billion times the mass of the sun. This makes understanding the speed and direction of kicks essential for tracking the formation of supermassive black holes.
In 2018, study co-author Juan Calderón Bustillo and his colleagues figured out exactly how to measure the natal kick based on these gravitational wave signals. But their model had to rely on simulations, as no black hole merger resulting in a recoil had been detected at that point.
Then, on April 12, 2019, the Advanced LIGO detectors in Louisiana and Washington State and the Virgo detector in Italy recorded the GW190412 picked up a signal resulting from two stellar-mass black holes merging: One 29.7 times as massive as the sun and the other 8.4 times as massive.
Despite taking place more than 2.4 billion light-years away from Earth, the researchers used two angles relative to Earth to determine where the kick sent the newborn black hole. It raced away from its birth site, likely a dense grouping of stars called a globular cluster, at an astonishing 111,600 miles per hour (179,600 kilometers per hour). This speed would be more than enough to enable it to escape the cluster and become a runaway black hole.
“This is one of the few phenomena in astrophysics where we’re not just detecting something,” Chandra said. “We’re reconstructing the full 3D motion of an object that’s billions of light-years away, using only ripples in spacetime.”
The team’s next steps will be to look for more black hole mergers to measure with both gravitational waves and visible light, a search that could yield deeper insights into how the cosmic monsters grow.