A new bump in the night: evidence of a new feature in the binary black hole mass distribution at $70~M_{\odot}$ from gravitational-wave observations

TL;DR

This study identifies a new feature at around 70 solar masses in the black hole mass distribution from gravitational-wave data, suggesting a possible hierarchical merger origin and expanding understanding of black hole populations.

Contribution

The paper introduces an alternative mass distribution model revealing a new high-mass feature at 70 solar masses, indicating a potential hierarchical merger population in gravitational-wave observations.

Findings

A second power law peaks at 30-35 solar masses.

A new feature at 65-70 solar masses suggests hierarchical mergers.

Maximum mass of the second power law is consistent with the pair-instability gap.

Abstract

We analyze the confident binary black hole (BBH) detections from the third Gravitational-Wave Transient Catalog (GWTC-3) with an alternative mass population model in order to capture features in the mass distribution beyond the Powerlaw + Peak model. We find that the peak of a second power law characterizes the $\sim 30-35~ M_\odot$ bump, such that the data marginally prefers a mixture of two power laws for the mass distribution of binary components over a Powerlaw + Peak model with a Bayes Factor $\log_{10}\mathcal{B}$ of 0.24. This result may imply that the $\sim 30-35~ M_\odot$ feature represents the onset of a second population of BBH mergers (e.g. from a dynamical formation channel) rather than a specific mass feature over a broader distribution. When an additional Gaussian bump is allowed within our power law mixture model, we find a new feature in the BH mass spectrum at $\sim65-70~M_\odot$ ($\log_{10}\mathcal{B}$ = 0.29 compared to Powerlaw + Peak). This new feature may be consistent with hierarchical mergers, and constitute $\sim3\%$ of the BBH population. This model also recovers a maximum mass of $58^{+32}_{-14}~M_\odot$ for the second power law, consistent with the onset of a pair-instability supernova mass gap.