Discovery of record-breaking black hole collision surprises astronomers

A team of international astronomers has caught the merger of two black holes of unprecedented masses creating yet another massive black hole — one that astronomers believed existed in theory but that had never been detected.

The two caught in the act were roughly 85 times and 66 times the mass of the sun (measured as solar masses). After the pair merged — producing a gravitational wave picked up by detectors — they created a black hole 142 times the mass of the sun.

"We've never seen something like this before," said Evan Goetz, a research associate at the University of British Columbia's physics and astronomy department and co-author of the paper published on Wednesday in the journals Physical Review Letters and Astrophysical Journal Letters.

"This is the first time any type of gravitational signal like this has been measured."

The discovery, called GW190521, was picked up using Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the United States and the Virgo interferometer in Italy on May 21, 2019.

So far, gravitational waves — ripples in space-time caused by highly energetic processes in space, such as the merging of pairs of black holes or neutron stars or a black hole and a neutron star — typically create a sort of "chirp" noise on the detectors.

LIGO/Caltech/MIT/R. Hurt IPAC]
LIGO/Caltech/MIT/R. Hurt IPAC]

But this one, the researchers say, created more of a short-lived "bang," producing only about five or six waveforms — which can be thought of as actual waves that oscillate. Comparatively, other gravitational-wave detections have produced hundreds of waveforms.

Due in part to its brief signal, astronomers had to ensure it wasn't noise that was causing the suspected detection. Now that it has been confirmed, it is the first of its kind — and is shedding light on an elusive member of the black hole family: intermediate-mass black holes.

Surprises abound with discovery

There are two special parts to the findings: one, the sizes of the pair of black holes, particularly the one that is 85 solar masses; and two, the final black hole itself.

Black holes are regions in space where gravity is so strong that nothing can escape them. But they're not all created equally.

According to theory, stars that are roughly 10 times the mass of the sun can die in a massive explosion — a supernova — that can produce a black hole. Stars that are roughly 65 times more massive are believed to destroy themselves. But stars that are more than 120 solar masses are believed to collapse directly into a black hole at the end of their lives.

So that means black holes between 65 and 120 solar masses shouldn't even exist. Yet this new discovery contains two that fall within that range. The one that is 85 solar masses is particularly intriguing because it falls right in the middle.

WATCH | Simulation of binary black holes merging:

Now to the final 142 solar-mass black hole.

There are stellar-mass black holes, which astronomers believe can go up to 10 to 100 times the mass of the sun. Then there are supermassive black holes, which can be found at the centre of most galaxies. These monsters can come in at millions or even billions of times the mass of the sun.

And while there have been theories about those black holes that are 100 solar masses or higher — called intermediate-mass black holes — none have been directly observed.

Until now.

"There's been no observational evidence prior to this discovery," Goetz said. "This is the first conclusive evidence for an intermediate-mass black hole."

Some hypotheses

There are two leading theories as to how these seemingly impossible things have come to be. One is that two stars could have merged to produce a black hole within the 65 to 85 range. The second is that multiple black holes could have merged within a dense star cluster, creating larger black holes of differing masses.

"This is kind of exciting," said Priya Natarajan, a theoretical astrophysicist and professor of astronomy and physics at Yale University in New Haven, Conn., whose main area of study is black holes. She was not involved with the research.

In a 2014 paper published in the journal Science, Natarajan and her colleague proposed that the creation of black holes similar to the 142 solar-mass one discovered last year could have come within a dense cluster of stars, much in the way the new discovery has been observed (though in her paper, the black hole was larger).

LIGO/Caltech/MIT/R. Hurt [IPAC]
LIGO/Caltech/MIT/R. Hurt [IPAC]

"The idea is one of the stars becomes a little black hole, and it starts wandering around and it's kind of fed by the fire hose of gas; it grows very fast," Natarajan said. "And then it becomes massive, and it grows to about 50 times and then it sinks to the centre.

"The second guy would roll around, but it wouldn't grow as fast because there wouldn't be as much gas left over. And so we predict that if you make two in that way, they cannot be the same mass, they will be different ... and that's what is seen now, so that's kind of exciting."

While the discovery is a first, the astronomers know that they need to increase their sample size to adequately explain the existence of intermediate-mass black holes.

"We need more observations of this type of signal. So, more mergers that yield intermediate-mass black holes will help us to understand ... how they're formed," Goetz said. "Black holes play a key role in so many aspects of astrophysics that we're only just now beginning to understand much more deeply with these observations."

And more importantly, they will help us understand how we got here, as black holes are key to creating much of what exists.

"It's part of the origin story of our universe," Natarajan said. "We may not have been here if our Milky Way did not have black holes."