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New Insights from LIGO/Virgo’s Merging Black Holes

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black holes in a globular cluster
Still from a simulation showing how black holes might interact in the chaotic cores of globular clusters. [Carl Rodriguez/Northwestern Visualization]

Since the first merger of two black holes detected in 2015, the LIGO/Virgo gravitational-wave detectors have observed a total of 47 confident collisions of black holes and neutron stars through the end of September 2019. What’s the big picture behind these events? The second gravitational-wave catalog is officially out — and the population statistics are in!

A New Catalog

In recent years, the Advanced LIGO detectors in Hanford, WA and Livingston, LA and the Advanced Virgo detector in Europe have kept a watchful vigil for ripples in spacetime that let us know that a pair of compact objects — black holes or neutron stars — has spiraled in and merged.

Plot showing masses of observed black hole and neutron star binary mergers. Plot includes black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange).

The rapidly expanding “stellar graveyard”, a plot that shows the masses of the different components of observed compact binary mergers included in the second gravitational-wave transient catalog (GWTC–2). [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

During LIGO’s first two observational runs (O1 in 2015–16 and O2 in 2016–17) the two LIGO detectors discovered 11 merger events. After a series of upgrades to the detectors, the system came back online in April 2019 for its third run (O3). In just the first 26 weeks of the run (O3a), LIGO/Virgo jointly found another 36 mergers!

In a new publication recently accepted to Physical Review X, the collaboration has released its second catalog of gravitational-wave events (GWTC–2), which includes data from O1, O2, and O3a. And in a companion publication in the Astrophysical Journal Letters, the team has now analyzed the broader set of all 47 mergers in the catalog, using population models to gain deeper insight into the binary properties and how these systems form and evolve.

Learning About Collisions

So what have we learned from the GWTC–2 population?

  1. Black-hole mass is more complicated than we originally thought.
    The merging black holes in O1 and O2 all had primary masses below 45 solar masses, consistent with the theory that black holes of ~50–120 solar masses shouldn’t be able to form. But O3a included several primaries above 45 solar masses, so we can’t model the primary mass distribution as a single power law with a sharp cutoff at 45 solar masses anymore. This may suggest that we’re looking at different populations of black holes that formed in different ways.
  2. Some black holes have spins that are misaligned with the angular momentum of the binary.
    Nine of the recent detections exhibit misaligned spins, which is another clue about their formation. Black hole binaries that form and evolve in isolated pairs are expected to have aligned spins, whereas black hole binaries that form dynamically — due, for instance, to interactions in clusters of stars or in the disk of an active galactic nucleus — should have isotropically distributed spins. The authors show that the spinning GWTC–2 population is consistent with 25–93% of black holes forming dynamically. That’s a large range, but what’s important is that this also indicates there’s more than one formation channel at work!
  3. The black hole merger rate probably increases with redshift.
    Plot showing how merger rate increases with redshift

    The modeled median merger rate density (solid curve) as a function of redshift suggests that the merger rate increases with redshift. Still, the increase is not as steep as the increase in the star formation rate (dashed line). [Abbott et al. 2021]

    Updated estimates suggest that binary black holes merge at a rate of 15–38 Gpc–3 yr-1 and binary neutron stars at a rate of 80–810 Gpc-3 yr-1. The merger rate appears to be higher at higher redshift, but this increase doesn’t quite parallel the known increase in star formation rate with redshift. An intriguing mystery!

A Smashing Good Time Ahead

These takeaways clearly represent a dramatic increase in our understanding of how and where black hole binaries form and evolve — but we still have so much left to learn! Luckily, there’s plenty more data ahead: the collaboration is now analyzing the remaining 5 months of data from O3, and the detectors are currently undergoing upgrades in preparation for O4, which is slated to begin in mid-2022.

Citation

“Population Properties of Compact Objects from the Second LIGO–Virgo Gravitational-Wave Transient Catalog,” R. Abbott et al 2021 ApJL 913 L7. doi:10.3847/2041-8213/abe949