An international collaboration of astronomers has observed the formation of a black hole with the mass of 142 suns, the first conclusive evidence of an intermediate-mass black hole. The black hole was the result of the most massive black hole merger ever detected with gravitational waves, which are ripples in spacetime that can be detected from Earth.
Not only did the merger produce the first example of a new kind of black hole, but one of the merging black holes possessed a “forbidden mass” that could not be explained by our usual understanding of how they form.
“I think it's remarkable that we got such a clear observation of 'Here's a black hole with can't be explained with our classic understanding of how stars collapse,'" said Christopher Berry, an astrophysics professor at Northwestern University and a LIGO Scientific Collaboration Editorial Board reviewer for the discovery paper.
The discoveries were enabled by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer, two of the world’s gravitational wave detectors. Companion papers published in the journals Physical Review Letters and Astrophysical Journal Letters on Wednesday described the signal, named GW190521.
Black holes come in various sizes—for instance, stellar-mass black holes range from several to tens of solar masses, while supermassive black holes are anywhere from hundreds of thousands to billions of solar masses. Stellar-mass black holes form when the force of gravity outweighs the force produced by nuclear fusion reactions at the centers of stars, causing the stars to collapse inward. However, stars heavier than 130 solar masses don’t end up producing black holes (which would have been 65 solar masses and up) when they collapse because of a phenomenon called pair instability.
When they analyzed the GW190521 signal, the researchers found that one of the black holes that combined to form the intermediate-mass black hole would have been around 85 solar masses, placing it squarely within the range of what Berry called a “forbidden mass.”
“The physics that goes into pair instability is well understood, so we need another way of making a black hole other than directly from a star collapsing,” he said.
The 85-solar mass black hole could be a second-generation black hole, formed when two smaller holes merged, or it could be the result of two stars merging and maintaining the core of just one of the stars. The black hole could have also been formed very early on in the universe’s history, making it a primordial black hole.
The GW190521 signal also points toward the solution of another astronomical mystery; namely, the formation of supermassive black holes, like the one at the center of our galaxy. Until this observation, there had been no direct evidence of an intermediate-mass black hole in the range between stellar and supermassive black holes.
Like the name implies, black holes do not emit any light, so previous research inferred their presence based on their effects on other nearby space objects. Because of LIGO and Virgo’s ability to detect gravitational waves, however, this collaboration was able to pick up on massive objects moving quickly, in this case a binary black hole.
“When you have a couple of black holes orbiting around each other, this is the perfect gravitational wave source,” Berry said, adding that the physics of this binary system is crucial for astronomers to understand. “Luke Skywalker looking up on Tatooine and seeing multiple suns setting is actually much more typical out in the universe than us and our lone star.”
If supermassive black holes form when stellar black holes combine, an intermediate-mass black hole may be a sort of “missing link,” though further research is needed to test this hypothesis and discover more black holes in this mass range.
September 14 will mark the five-year anniversary of the first direct observation of gravitational waves. Berry said that the pace at which LIGO and Virgo have aided discoveries since makes today a “fantastic time to be in gravitational wave astronomy.”
“I've never ceased to be surprised by just how rapidly we are making these unprecedented discoveries with gravitational waves.”