Astronomers have detected the most powerful, most distant and most perplexing collision of black holes yet using gravitational waves. Of the two behemoths that fused when the Universe was half its current age, at least one — weighing 85 times as much as the Sun — has a mass that was thought to be too large to be involved in such an event. And the merger produced a black hole of nearly 150 solar masses, the researchers have estimated, putting it in a range where no black holes had ever been conclusively seen before.
“Everything about this discovery is mindboggling,” says Simon Portegies Zwart, a computational astrophysicist at Leiden University in the Netherlands. In particular, he says, it confirms the existence of ‘intermediate mass’ black holes: objects much more massive than a typical star, but not quite as big as the supermassive black holes that inhabit the centres of galaxies.
Ilya Mandel, a theoretical astrophysicist at Monash University in Melbourne, Australia, calls the finding “wonderfully unexpected”.
The event, described in two papers published on 2 September1,2, was detected on 21 May 2019, by the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and by the smaller Virgo observatory near Pisa, Italy. It is named GW190521 after its detection date.
Since 2015, LIGO and Virgo have provided new insights into the cosmos by sensing gravitational waves. These ripples in the fabric of space-time can reveal events such as the mergers of black holes that would not normally be visible with ordinary telescopes.
From the properties of the gravitational waves, such as how they change in pitch, astrophysicists can estimate the sizes and other features of the objects that produced them as they were spiralling into each other. This has revolutionized the study of black holes, providing direct evidence for dozens of these objects, ranging in mass from a few to about 50 times the mass of the Sun.
These masses are consistent with black holes that formed in a ‘conventional’ way — when a very large star runs out of fuel to burn and collapses under its own weight. But the conventional theory says that stellar collapse should not produce black holes between about 65 and 120 solar masses. That’s because towards the end of their lives, stars in a certain range of sizes become so hot in their centres that they start converting photons into pairs of particles and antiparticles — a phenomenon called pair instability. This triggers the explosive fusion of oxygen nuclei, which rips the star apart, completely disintegrating it.
In their latest discovery, the LIGO and Virgo detectors sensed only the last four ripples produced by the spiralling black holes, with a frequency that rose from 30 to 80 Hertz within one-tenth of a second. While relatively smaller black holes continue to ‘chirp’ up to higher frequencies, very large ones merge earlier, and barely enter the lower end of the frequency range to which the detectors are sensitive.
In this case, the two objects were estimated to weigh around 85 and 66 solar masses. “This is quite neatly in the range one would expect the pair-instability mass gap should be,” says LIGO astrophysicist Christopher Berry at Northwestern University in Evanston, Illinois.
Selma de Mink, an astrophysicist at Harvard University in Cambridge, Massachusetts, puts the cut-off for pair instability even lower, perhaps at 45 solar masses, which would push the lighter of the two objects firmly into the forbidden zone, too. “For me, both black holes are uncomfortably massive”, she says.
Unconventional black holes
To explain their observations, the LIGO researchers considered a range of possibilities, including that the black holes had been around since the beginning of time. For decades, researchers have conjectured that such ‘primordial’ black holes could have spontaneously formed in a broad range of sizes shortly after the Big Bang.
The main scenario the team contemplated is that the black holes got so large because they were themselves the result of earlier black-hole mergers. Black holes resulting from stellar collapse should teem inside dense stellar clusters, and in principle they could undergo repeated mergers. But even this scenario is problematic because, following a first merger, the resulting black hole should typically get a kick from the gravitational waves and eject itself from the cluster. Only in rare cases would the black hole stay in an area where it could undergo another merger.
Successive mergers would be more likely if the black holes inhabited the crowded central region of their galaxy, de Mink says, where gravity is strong enough to prevent recoiling objects from shooting out.
It is not known in which galaxy the merger happened. But in roughly in the same region of the sky, a team of researchers spotted a quasar — an extremely bright galactic centre powered by a supermassive black hole — undergoing a flare around a month after GW1905213. The flare could have been a shockwave in the quasar’s hot gas produced by the recoiling black hole, although many astronomers are cautious to accept that the two phenomena are related.
This is the second time this year that the LIGO–Virgo collaboration has waded into a ‘forbidden’ mass range: in June, they described a merger involving an object of about 2.6 solar masses — typically considered too light to be a black hole but too massive to be a neutron star4.
Nature 585, 171-172 (2020)