Gravitational waves from merging black holes encode the mass and spin of the original black holes in the system. These properties can be heavily influenced by the interactions the black hole binary experiences prior to merging, especially when these systems are within dense star clusters. A recent study explores how black hole binaries are impacted by stellar collisions in cluster environments and what that may mean for the gravitational waves we detect here on Earth.
Gravitational Waves from Black Hole Binaries
Animation of a black hole binary merger. As the black holes merge, they create gravitational waves, warping the fabric of spacetime. Click to enlarge. [LIGO/T. Pyle]
Multiple formation mechanisms have been proposed that are sensitive to the characteristics of the black holes in the binary. Importantly, the spin (rotation) of the black holes can be used to distinguish how the system likely formed. From theory, stellar-mass black holes likely form with little spin but will spin up as they interact with stars and other black holes throughout their lifetimes. Interactions causing spin-up are more likely to happen in crowded areas like star clusters, so understanding how binary black holes form and evolve within these dense environments will enable comparison to LVK observations.
Simulations of Clusters
To explore how collisions and close encounters with stars influence black hole properties within dense star clusters, Fulya Kıroǧlu (CIERA; Northwestern University) and collaborators performed eight simulations across a range of cluster models. Varying characteristics such as metallicity, cluster radius, and binary fraction for massive stars, the authors explore how the black holes in each cluster evolve over 12 billion years.
Through these simulations, the authors track the number of black hole–star collisions and find that, depending on the cluster properties, black holes and stars undergo collisions at early and late times in the cluster’s life. For stellar clusters with high-density cores and more massive star binaries, black holes are more likely to collide with high-mass stars, increasing the rate at which spinning black holes are formed. Additionally, the metallicity of the star cluster can significantly impact how many black hole–star interactions occur. For merging black hole binaries in star clusters, especially those with high metallicity, over 50% had at least one black hole that had spun up after a previous collision with a star. These results build an expected distribution of binary black hole merger spins that can be compared to gravitational wave detections.
Effective spin versus primary black hole mass distribution. The results from the simulations in this study are shown in purple and cyan. The background points show the predicted distribution based on LVK observations. Click to enlarge. [Kıroǧlu et al 2025]
Comparing to Observations
What do the simulation results mean when it comes to LVK observations? The authors compare the distribution of binary black hole spin and mass for all mergers in their models to the predicted distribution based on LVK observations. Their modeling reproduces the predicted trends from gravitational-wave data, and recent observations suggest a population of binary black hole mergers with high spins consistent with this work.
As LVK continues to detect gravitational waves, further simulations with more detailed hydrodynamic models will be necessary in order to uncover the full range of possible outcomes of binary black hole–star interactions. This work serves as a critical step in revealing the origins of gravitational waves.
Citation
“Black Hole Accretion and Spin-up through Stellar Collisions in Dense Star Clusters,” Fulya Kıroǧlu et al 2025 ApJ 979 237. doi:10.3847/1538-4357/ada26b