Gravitational waves reveal black hole merger mass distributions and origins.

Analyses of binary black hole mergers reveal a peak in merger rates around 30 solar masses, followed by a rapid decline. Observed distributions of primary mass, mass ratio and spin favour near equal-mass systems with positive spin and correlate with cosmic star formation. Current formation models struggle to explain these features.

The observed prevalence of binary black hole mergers featuring component masses around 35 solar masses presents a significant puzzle for astrophysicists, challenging existing models of stellar evolution and black hole formation. Soumendra Kishore Roy, Lieke A. C. van Son, and Will M. Farr, alongside colleagues, address this anomaly in their research, detailed in the article “A Mid-Thirties Crisis: Dissecting the Properties of Binary Black Hole Sources Near the 35 Solar Mass Peak”. Their analysis of gravitational wave data reveals a distinct population of merging black holes exhibiting specific characteristics in primary mass, mass ratio (the ratio of the masses of the two black holes), effective spin (a measure of the combined angular momentum), and redshift (a measure of distance and the expansion of the universe). The study’s findings suggest that current theoretical frameworks struggle to account for the observed properties of these systems fully, indicating a need for refinement or the development of entirely new models of black hole formation.

Gravitational-wave observations consistently reveal a concentration of binary black hole (BBH) mergers around 30 solar masses, prompting a detailed investigation into the formation channels responsible for creating this population. Researchers infer population-level distributions of key parameters – primary mass, mass ratio, effective spin, and redshift – to differentiate between competing formation models and understand the underlying physics. The study demonstrates a clear trend: the merger rate increases with primary mass, peaking around 30 solar masses before experiencing a substantial, order-of-magnitude decline, indicating a preference for black hole mergers within this specific mass range. This population exhibits a slight preference for equal-mass mergers, suggesting that mergers of similarly sized black holes are more common.

Effective spin, a parameter describing the alignment of the black holes’ rotation with their orbital motion, is distributed around zero, with a statistically significant deviation from zero confidently excluded, indicating that the spins of the merging black holes are generally aligned with their orbital angular momentum. Importantly, the peak population analysed does not show significant correlations, suggesting that lower-mass systems primarily drive the anti-correlation observed in the broader BBH merger catalogue, highlighting the unique characteristics of this population. Analysis of redshift data, which measures the distance and thus age of the mergers, indicates the merger rate evolves in line with the cosmic star formation rate, supporting the notion that these mergers occur within star-forming galaxies and are linked to the birth of stars. However, comparison with predictions from various formation channels reveals significant discrepancies, challenging existing models of black hole formation and evolution.

Common variations of the pair-instability supernova scenario, a process where massive stars collapse directly into black holes, and hierarchical merger models, where black holes form through multiple mergers, both fail to reproduce the observed features of the population, indicating a need for new theoretical frameworks and a reevaluation of the processes that govern the birth and evolution of binary black holes. The observed population presents a significant challenge to current theoretical frameworks, necessitating further investigation into alternative formation pathways and a refinement of existing models to reconcile theoretical predictions with observational data. Researchers continue to explore new scenarios and refine existing models to explain the observed population of merging black holes and unravel the mysteries of their formation and evolution.