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The merger of middleweight black holes churned up gravitational waves that were detected on Earth.

Deborah Ferguson, Karan Jani, Deirdre Shoemaker, Pablo Laguna, Georgia Tech, MAYA Collaboration

Most massive black hole merger yet puzzles astronomers

Far away in the depths of space, two black holes spiral toward each other and merge. Powerful gravitational waves from that dance of death race across the cosmos until their ripples reach three giant detectors on Earth: two with the U.S.-based Laser Interferometer Gravitational-wave Observatory (LIGO) and Europe’s Virgo detector in Italy.

The detectors have sensed dozens of such cataclysms over the past 5 years, but the one on 21 May 2019 was different. Not only was it the most powerful and distant merger ever seen, but the resulting black hole also belongs to a class of long-sought middleweight black holes, members of the LIGO-Virgo collaboration report today in two new studies. Puzzlingly, however, the two black holes that merged are heavier than expected: Their masses fall in a gap in which theorists believe it is impossible to make a black hole via the usual route of a collapsing star.

Stellar-class black holes are typically created when a large star runs out of its nuclear fuel and the churning engine of light and heat stops. Without that outward pressure, the star’s outer layers collapse under gravity, triggering a colossal supernova and leaving behind a black hole. But in the very biggest stars, the collapse is even more catastrophic, causing a runaway thermonuclear explosion that destroys the star and leaves nothing behind. Theoretically, that means there should be a cutoff in black hole mass at about 65 solar masses.

Until May 2019, black hole mergers detected by LIGO and Virgo largely supported that mass cutoff. Then came the event known as GW190521, which lasted just one-tenth of 1 second. It wasn’t spotted by the usual algorithms that scan for binary mergers (which typically last several times longer), but was picked up by a separate pipeline that looks for “things that go bang,” says Nelson Christensen, a physicist at the Cote d’Azur Observatory in Nice and a member of the LIGO-Virgo team.

Although the signal was short—just four up-and-down wave cycles—the team could still analyze it, parsing out its amplitude, its shape, and how its frequency changed over time. “It was very difficult to interpret,” says team member Alessandra Buonanno, director of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute). “We spent a lot of time persuading ourselves to trust what we’d found.”

In two papers published today—one describing the detection in Physical Review Letters, and one interpreting the data in The Astrophysical Journal Letters—the joint LIGO-Virgo team says the model that best fits the data is of two black holes—weighing in at about 66 and 85 solar masses—merging into a black hole of 142 Suns. The remaining eight solar masses would have been converted into gravitational wave energy. “It was quite substantially bigger than anything we’d seen,” Christensen says.

A black hole with 142 solar masses instantly puts it into a class of its own. Whereas astronomers have long known of smaller black holes and of the giants in galactic centers made of millions or billions of Suns, those of medium size—from 100 to 100,000 solar masses—have been conspicuously absent. Astronomers believe they are needed as building blocks for the supermassive black holes, and there is indirect evidence for their existence, but this may be the most convincing sighting yet, albeit right at the bottom of the range. “This is just a hint that there is something in this range of masses,” says astrophysicist Avi Loeb of Harvard University who was not involved in the study.

Perhaps more interesting to astrophysicists are the origins of the two merging black holes. The lighter one is right on the cusp of the mass gap, so it could well have formed from a single gargantuan star. But 85 solar masses is hard to explain away. “It’s exciting because it was unexpected,” Loeb says. “The mass gap was robust, but now the door is open to new models.”

In their interpretive paper, the team looked at many possible explanations. The black holes could be primordial, having hung around since the maelstrom of the early universe before the first stars were born. Or they could have been small black holes, with a merger that was magnified by gravitational lensing. Or perhaps—more exotically—the ripples came from cosmic strings, hypothetical defects in the vacuum left over from the big bang. But none of these explanations fitted the data as well as a pair of merging heavyweights. So, the team fell back on “good old Occam’s razor,” Christensen says: The simplest explanation is probably correct.  

Loeb believes the heavyweights are probably “multigenerational," in which smaller black holes in dense star-forming areas merge multiple times to produce masses above the cutoff. Galaxies are often surrounded by dense clumps of stars called globular clusters. These can contain hundreds of thousands of ancient stars: ideal breeding grounds for black holes. As the black holes sink toward the center of the globular cluster, they are more likely to merge with others. “These environments are specialized, which is why we are only finding them now,” he says, after LIGO and Virgo have sensed more than 60 mergers.

But the clusters are likely to contain black holes of varying masses, and lopsided mergers produce asymmetric blasts that can kick the new black hole out of the cluster at up to 1000 kilometers per second. For clusters to be nurseries for black holes in the mass gap, the recoils need to be low and the clusters must be massive enough to keep them from escaping, Loeb says.

LIGO and Virgo are being upgraded and are set to restart observations in 2022 with increased sensitivity, allowing them to survey three times as much of the cosmos. Finding more such heavyweight mergers will “teach us about the astrophysics of such stellar nurseries,” Loeb says. “The more events we have, the more clues about their origins.”