The alert caught Zsuzsanna Márka’s attention immediately. Whenever the U.S.-based Laser Interferometer Gravitational-wave Observatory (LIGO) and its European counterpart Virgo detect a potential gravitational-wave event, an automatic notice is quickly dispatched to members of the collaboration.

Soon after receiving the notification for such an event on May 21, 2019, Márka, an astrophysicist at Columbia University, was on the team’s internal Slack channel, sending an excited message to her husband, Szabolcs Márka, who also works with the LIGO group at Columbia, and to Imre Bartos of the University of Florida.

“This one was very special,” Zsuzsanna Márka recalls. “I noticed [it involved] high masses right away.”

Details from LIGO-Virgo’s instruments indicated they had captured the signal of two behemoth black holes spinning around each other and merging into a single entity some 17 billion light-years away. The progenitor leviathans respectively weighed around 85 and 66 times the sun’s mass, the heaviest pair detected to date by the facilities. And the subsequent black hole clocked in at an astounding 142 solar masses.

The breaking of records was not the only reason for the scientific elation. LIGO and Virgo, which use ultrasensitive quantum-mechanical sensors to identify ripples in the fabric of spacetime generated by cosmic cataclysms, had never before found black holes in what is known as the “intermediate” range between 100 and 1,000 solar masses. Astronomers have not previously seen an unambiguous example of such black holes—and are not even sure how they might be created.

But within the LIGO-Virgo data lay tantalizing clues to the environment in which these heavyweights formed, providing researchers with their best real-world observations of an object that had hitherto been largely theoretical. Those in the field know that the finding is a harbinger of things to come, and they are looking forward to soon having many more such signals to analyze.

“I see it as a threshold event—it’s just the tip of the iceberg,” says Priyamvada Natarajan, an astrophysicist at Yale University, who studies black hole formation but was not involved in the work. “I’m so stoked.”

A black hole typically arises from the death of a massive star, which ends its life in a spectacular supernova explosion. As the star disintegrates, its dense core collapses into an object so compact and heavy that not even light can escape its gravitational pull—a black hole. The more massive the original star is, the more massive its subsequent remnant will be, at least until a certain point.

The death throes of extremely heavy stars, those greater than 130 times the sun’s mass, include an extra twist. Temperatures get so hot in their core that photons of light begin generating pairs of electrons and their antiparticles, positrons. This change leads to a drop in the outward “radiation pressure” that the photons exert, causing the bulky outer layers to collapse inward with such ferocity that the entire core detonates in a thermonuclear explosion powerful enough to annihilate the star. No black hole relic can be left behind in that wake of stellar devastation, leading to a theoretical upper limit to black holes’ size: around 65 solar masses.

The problem is that researchers know black holes with millions to billions of times the mass of the sun are lurking in the centers of pretty much every known large galaxy. So where exactly did these monsters come from?

Many stars form with a nearby stellar companion, and the two will orbit each other for their entire lives. If both of them are high-mass stars, they might explode at roughly the same time and leave behind a pair of black holes. These black holes can gravitationally attract and slowly spiral toward each other, and their eventual merger will send out copious gravitational waves traveling in all directions at the speed of light. LIGO-Virgo was built in part to capture such signals, and the collaboration’s instruments have so far seen more than 10 such mergers, each involving black holes ranging from roughly five to 50 times the sun’s mass.

But if two black holes can merge, then perhaps the resulting entity can find another black hole and repeat the process. “It’s like a little assembly machine,” Szabolcs Márka says. “You take a black hole and merge it, make a bigger black hole and merge it.” Such so-called hierarchical mergers had been previously theorized but never seen.

Although the event in May 2019, which has been labeled GW190521, lit up LIGO-Virgo’s sensors for less than a tenth of a second, it contained enticing information about the merging black hole pair. Specifically, the detectors found that each of the black holes was spinning around like an enormous top, a property that LIGO-Virgo had seen in only one black hole merger. This observation alone made GW190521’s black holes unusual. But researchers were even more intrigued to see that their spins were not aligned—a telltale sign that the compact objects had not known each other for very long.

When two stars form two black holes, gravity acts as a harmonizing force, bringing each entity in line with its partner. The black holes should both spin in the same direction as their orbital path around each other, much like how the moon spins around its own axis in the same direction that it orbits Earth. The misaligned spins of GW190521’s gigantic black holes hint that gravity did not have a great deal of time to work its coordinating magic before they merged. That idea suggests they did not originally form together but rather lived in an environment thick with other black holes.

“There is one special kind of place where this can happen,” Bartos says. “And that’s in the centers of galaxies, where smaller black holes tend to congregate in the vicinity of a supermassive black hole.”

A lurking supermassive black hole makes a galactic center rather like the bottom of a well. Other heavy objects, such as stellar-mass black holes, will fall in the direction of its hefty attraction. Because GW190521 occurred so far away, it comes from a time when the universe was only half its current age, an era when many galaxies were blazing brightly as their central supermassive black holes were vigorously consuming gas and dust and belching out energy. Such swirling active galactic nuclei (AGNs), as they are known, would have been hotspots of commotion where smaller black holes could have met new partners and merged, explaining the new LIGO-Virgo event.

Such a picture is not guaranteed to be the situation with GW190521, but much of the evidence points in this direction. It is even possible that the heavier object in the pair, at 85 solar masses, formed from its own prior merger, Bartos says, although it cannot be ruled out that this extra-bulky black hole was created by some exotic, unknown process. “That’s one of the beauties and difficulties of this field,” he adds. “We are working with complex systems that are very far away.” The team’s findings appeared last September in two papers in Physical Review Letters and the Astrophysical Journal Letters.

Natarajan, who has been working on models to form black holes between 100 and a million times the sun’s mass, says that the results are exciting because “they directly give you the stepping-stone to supermassive black holes.” Astronomers know that supermassive black holes must have gone through such a middle stage, she adds, but until now evidence of that period had been elusive.

LIGO-Virgo’s facilities are currently closed because of the ongoing COVID-19 pandemic. Once the instruments come back online, however, researchers are eager to see whether they will find more events involving black holes in this intermediate mass range. The fact that it has taken this long to see these first results suggests that such mergers are somewhat rare, though not exceedingly so. Upgrades to the observatories should give scientists a clearer view of the moments leading up to the merging events, helping to determine whether they occurred in an AGN or some different environment.

Particularly helpful data could come from other telescopes that hunt for flashes of light whenever they get a LIGO-Virgo alert. Studying the optical, ultraviolet or infrared counterparts to gravitational-wave events gives astronomers multiple pathways to understanding their details. Just after the May 2019 detection, the Zwicky Transient Facility at the Palomar Observatory in California spotted an optical flicker in the vicinity of a distant AGN, but it remains unclear whether the two results are related.

Nevertheless, people in the field are happy to be at this turning point. Although GW190521 will go down in history as the first intermediate black hole definitively discovered, researchers are confident that they will soon have plenty of other examples to learn from.

“From this moment on, we have agents that are teaching us about something we couldn’t reach in any other way,” Szabolcs Márka says. “Before this event, it was all a dream. Now it has become a testable theory.”