Compactness peaks: An astrophysical interpretation of the mass distribution of merging binary black holes

¹ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bd de l’Observatoire, 06300 Nice, France
² Laboratoire Artemis, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bd de l’Observatoire, 06300 Nice, France

^⋆ Corresponding authors; shanika.galaudage@oca.eu

Received: 25 July 2024
Accepted: 22 October 2024

Abstract

With the growing number of detections of binary black hole (BBH) mergers, we are beginning to probe structure in the distribution of mass. A recent study proposes that the isolated binary evolution of stripped stars naturally gives rise to the peaks at ℳ ∼ 8 M_⊙ and 14 M_⊙ in the chirp-mass distribution and explains the dearth of black holes (BHs) in the mass range of ℳ ≈ 10 − 12 M_⊙. The gap in chirp mass results from an apparent gap in the component-mass distribution within m₁, m₂ ≈ 10 − 15 M_⊙ and the specific pairing of these BHs. This component-mass gap results from variation in the core compactness of the progenitor, where a drop in compactness as a function of carbon–oxygen core mass means that BHs are no longer formed from core collapse. We develop a population model motivated by this scenario to probe the structure of the component-mass distribution of two populations of BBHs: one population consisting of two peak components, representing BHs formed in the compactness peaks, and another population with a power-law component to account for any polluting events, that is, binaries that may have formed from different channels (e.g. dynamical). We perform hierarchical Bayesian inference to analyse the events from the third gravitational-wave transient catalogue (GWTC-3) with our population model. We find that there is a preference for the lower-mass peak to drop off sharply at ∼11 M_⊙ and the upper mass peak to turn on at ∼13 M_⊙, in line with predictions in the literature. However, we find no clear evidence for a gap. We also find mild support for a scenario where the two populations have different spin distributions. In addition to these population results, we highlight observed events of interest that differ from the expected population distribution of compact objects formed from stripped stars.