The spin magnitude of stellar-mass binary black holes evolves with the mass: evidence from gravitational wave data

TL;DR

This study reveals a correlation between black hole mass and spin in stellar-mass binary black holes, indicating different formation channels, with higher mass black holes showing higher spins, consistent with hierarchical merger models.

Contribution

The paper provides the first comprehensive analysis of mass-spin correlations in BBHs using hierarchical Bayesian methods on gravitational wave data.

Findings

Strong evidence for a mass-spin correlation in BBHs.

A transition in spin distribution occurs at 40-50 solar masses.

High-mass BBHs likely originate from hierarchical mergers.

Abstract

The relation between the mass and spin of stellar-mass binary black holes (BBHs) has been proposed to be a smoking gun for the presence of multiple formation channels for compact objects. First-generation black holes (BHs) formed by isolated binary stellar progenitors are expected to have nearly aligned small spins, while nth-generation BBHs resulting from hierarchical mergers are expected to have misaligned and higher spins. Leveraging data from the third observing run O3 (GWTC-2.1 and GWTC-3), we employ hierarchical Bayesian methods to conduct a comprehensive study of possible correlations between the BBH masses and spins. We use parametric models that either superpose independent BBH populations or explicitly model a mass-spin correlation. We unveil strong evidence for a correlation between normalized spin magnitudes and masses of BBHs. The correlation can be explained as a transition from a BBH population with low spins at low masses and higher spins for higher masses. Although the spin magnitude distribution at high masses lacks robust constraints, we find strong evidence that a transition between two BBH populations with different spin distributions should happen at 40-50 $M_{\odot}$. In particular, we find that the population of BBHs above 40-50 $M_{\odot}$ should compose the $\sim 2 \%$ of the overall population, with a spin magnitude $\chi$ peaking around 0.7, consistently with the fraction of nth-generation BBHs formed by hierarchical mergers in the latest state-of-the-art BBH genesis simulations.