Big Black Holes Dominate New Gravitational-Wave Catalog

The LIGO and Virgo collaborations recently released their second official catalog of gravitational-wave detections, raising the total number of collisions from 11 to 50. Of these events, nearly all were pairs of black holes merging.

The new 39 detections come from the first six months of the collaboration’s third observing run, or O3, which ran from April to October 2019. They don’t include additional candidates teased out of previous runs’ data by independent teams. Twenty-six of the 39 had already been announced as public alerts, caught by automated systems and quickly confirmed by researchers. Scientists found the remaining 13 when combing through the data in more sensitive follow-up searches.

Several of the O3 events have already made headlines, announced on their own over the course of 2020. These have included black holes with mismatched sizes and a collision potentially embedded in gas.

Unlike those individual announcements, the catalog enables us to look at the detections as a cohort. The objects involved span a range of masses, from just above 1 solar mass to about 90 Suns. The closest was 500 million light-years away; the farthest, a fair fraction to the edge of the observable universe, looking back roughly 7 billion years. Only one of the new 39 events may have been two neutron stars crashing into each other, but unlike with the spectacular event GW170817, astronomers found no traces of a burst of light — the possibility is based on the objects’ masses.

What’s really eye-catching about the new catalog is just how many of the black holes are big. More than half of the mergers involved at least one black hole weighing 30 solar masses or more. Before LIGO caught its first merger back in 2015, most astronomers didn’t think they’d find black holes that big — the predictions existed but weren’t widely bandied about.  

But now, scientists have seven cataclysms with a black hole weighing at least 50 Suns, with one whopper tipping the scale at about 90 solar masses.

These high-mass systems are one of the most important discoveries in the new catalog. Astronomers had predicted that there should be a black hole desert between about 50 and 130 solar masses (exact values depend on whom you ask). This no-man’s land would exist because as a massive star reaches the advanced stages of life, madly fusing carbon in its core, conditions arise that make the core unstable and cause it to contract violently. If the core is above about 130 solar masses, that contraction will be unstoppable, turning the core into a black hole. But at lower core masses, fusion can proceed in an explosive enough way to reverse the implosion and tear the star apart, leaving nothing behind.

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To see so many black holes in the gap was definitely surprising, says LIGO scientist Maya Fishbach (Northwestern University). It’s easier for the gravitational-wave observatories to sense the smashups of big black holes than small ones. But when the researchers take the instruments’ limitations into account and extrapolate what the whole population would look like if they could detect everything perfectly, they estimate that about 3% of systems contain a black hole heftier than 45 solar masses — higher than the estimate from their first catalog, which was 1%.

It’s unclear what’s going on. The collaborations do see signs that the number of black holes drops off above 40 solar masses. “It could be that there’s a subpopulation that’s actually contaminating the gap,” Fishbach says — second-generation black holes made not by dying stars but subsequent mergers. Or, perhaps the cutoff is at a higher mass or is gradual, with stars finding various ways to stave off collapse.

Stan Woosley (University of California, Santa Cruz) is among the astronomers diving into the second possibility. By making reasonable changes to stars’ fusion rates, their rotations, and how much stuff they can hold onto before death or snatch later from companions, it’s possible to nudge the gap’s lower boundary up to about 65 solar masses, he says — explaining all but one of the gravitational-wave sources.

A recent laboratory study by an international team also suggests that helium fusion might proceed about 35% more efficiently in stellar cores. If so, that could help build up carbon and slow down collapse, making it harder for the star to obliterate itself at lower masses.

But no one knows quite what to do with the 90-solar-mass-black hole. “That’s right in the middle of the forbidden zone,” Woosley says. “If you took just that one out, the rest, I think, would be accommodated.”

How to Make a Black Hole Binary

One way to find out big black holes’ provenances is their spins. Black holes created by mergers generally spin at about 70% of their maximum rate — in fact, all the remnant black holes in the O3 catalog have final spins clustered around this value. Conversely, a black hole made by a dying star will likely spin much slower, a prediction validated by a black hole involved in one of the O3 catalog’s mergers, GW190814.

The spin’s tilt also matters. Although astronomers haggle about exceptions, black holes born as fraternal twins from stars already in a binary will likely spin like upright tops around each other. Black holes that adopt each other later, however, are more likely to have a random assortment of inclinations.

But it’s still hard to measure objects’ individual spins before the merger, Fishbach says. Instead, scientists have to look at them as an ensemble and make statistical inferences. From that analysis, the collaborations can tell that some pre-merger black holes are definitely spinning, and about a third of them are either rolling on their sides through their orbits or spinning upside-down compared to the direction they circle their partners.  

That suggests — as a very preliminary picture, mind you — that roughly a third of the colliding black holes caught by LIGO and Virgo paired up in, say, a dense star cluster or the fluffy gas disk around a supermassive black hole, instead of being born together as a binary from the get-go.  

More Data Coming

The new catalog and the companion papers include many other intriguing findings, from confirmations of Einstein’s theory of gravity to evidence that the black hole merger rate was about twice as high 7 or 8 billion years ago as it is today. (That’s expected, since star formation peaked in the universe about 10 billion years ago.)

There are also signs that far fewer low-mass black holes exist. The smallest black hole binary detected involved objects of 9 and 5 solar masses. Although there’s at least one object smaller than that that might be a black hole, there does seem to be a dearth of lightweights, Fishbach says.

“But it’s a messy picture,” she cautions. “We’re not sure where the gap is, and we’re not sure if the gap is empty.”

This picture will become clearer with data from the second half of O3, which ran from November 2019 until March 2020. There are 23 public alerts for possible gravitational-wave events from that run, but it’ll probably be another six to 12 months before the official analysis comes out.

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References:

The LIGO Scientific Collaboration and the Virgo Collaboration. “Population Properties of Compact Objects from the Second LIGO-Virgo Gravitational-Wave Transient Catalog.” Posted to arXiv.org on October 27, 2020.

The LIGO Scientific Collaboration and the Virgo Collaboration. “GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run.” Posted to arXiv.org on October 27, 2020.

The LIGO Scientific Collaboration and the Virgo Collaboration. “Tests of General Relativity with Binary Black Holes from the Second LIGO-Virgo Gravitational-Wave Transient Catalog.” Posted to arXiv.org on October 27, 2020.

T. Kibédi et al. “The radiative width of the Hoyle state from γ-ray spectroscopy.” Physical Review Letters. October 27, 2020. Preprint here.