Can Gravitational Waves Tell Us How Binary Black Holes Form?

How do binary black holes form? Are the two components born together as stars, or do they find each other only after evolving into black holes? Gravitational waves may hold the answer.

Imprints of Formation History

Gravitational-wave detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer have allowed us to eavesdrop on the mergers of black holes across the universe. With dozens of black hole mergers detected so far, we can start to question how two black holes end up in a binary system in the first place.

Two main pathways are expected. In the first scenario, two stars, coupled since their formation, engage in a slow gravitational dance over billions of years as they evolve into black holes, grow closer, and merge. In the second scenario, two black holes, born apart, become gravitationally entangled in a dynamic environment like a dense star cluster.

gravitational wave forms

An idealized example of gravitational wave forms for a binary black hole system with perfectly circular orbits (e=0; black) and very elongated orbits (e=0.5; red). Click to enlarge. [Abbott et al. 2019]

These two formation pathways affect how eccentric, or elongated, the orbits of the black hole binary pair will be in the moments before they merge. In the isolated origin scenario, the black holes emit gravitational waves as they slowly slink toward each other, leading their orbits to become circular before they merge. In the dynamic origin scenario, on the other hand, black holes that become entangled are driven to merge quickly, before their orbits can circularize. The eccentricity of the binary system alters the gravitational waves released just before the merger, giving astronomers a way to track down the origins of these systems.

eccentricity probability distributions for black hole binaries

Violin plot showing probability distributions for the eccentricities of the 26 binary black hole mergers analyzed in this study and 10 mergers that were previously analyzed. The wider the violin, the more likely the eccentricity. Most events are weighted toward e=0, though two events (GW190620A and GW190521A) show clear signs of nonzero eccentricity. Click to enlarge. [Romero-Shaw et al. 2021]

Examining Eccentricity

A team led by Isobel Romero-Shaw (Monash University and ARC Center of Excellence for Gravitational Wave Discovery, Australia) performed a statistical analysis of gravitational-wave signals from 26 binary black hole mergers in the LIGO/Virgo catalog to determine the most likely eccentricity — and therefore the most likely origin — for each merging system.

Romero-Shaw and collaborators found that while the majority of the events analyzed likely had circular orbits, two events showed clear signs of eccentricity, with 50% of their probability distributions falling above an eccentricity of 0.05. The team’s results suggest that 27% or more of the binary black holes in the LIGO/Virgo catalog formed dynamically, likely in a dense cluster environment.

Emerging Possibilities

ground-based image of a star cluster

Black hole binaries in young star clusters, like NGC 3293 shown here, may resemble those formed in dense clusters or isolated environments. [ESO/G. Beccari]

Aside from the systems that were clearly eccentric, Romero-Shaw and collaborators found a further 10 events that showed hints of eccentricity but were still consistent with circular orbits. It’s not yet clear what these marginal detections mean, since random fluctuations could cause perfectly circular binaries to appear eccentric. If these marginal cases truly are eccentric, dense star clusters may not be sufficient to produce the number of observed dynamically assembled systems. This may mean that eccentric black hole binaries can form in other places, such as young open star clusters, which might serve as a gravitational middle ground between dynamic and isolated environments, or the disks surrounding active galactic nuclei.

There’s still much to learn about merging black holes, and luckily our gravitational-wave detectors are hard at work. Hopefully, future detections of black hole mergers help us discern how these systems form!


“Signs of Eccentricity in Two Gravitational-wave Signals May Indicate a Subpopulation of Dynamically Assembled Binary Black Holes,” Isobel Romero-Shaw et al 2021 ApJL 921 L31. doi:10.3847/2041-8213/ac3138