The Heavier the Faster: A Sub-population of Heavy, Rapidly Spinning and Quickly Evolving Binary Black Holes

TL;DR

This paper introduces the 'dimensionless net spin' to analyze binary black hole spins, revealing two distinct populations and suggesting dynamical formation processes are significant for high-mass systems.

Contribution

It proposes a new spin parameter and demonstrates its effectiveness in distinguishing black hole populations, challenging existing binary formation models.

Findings

Two Gaussian components fit the spin distribution better than one.

Rapidly spinning, high-mass black holes evolve faster with redshift.

Dynamical formation processes likely contribute to high-mass black hole binaries.

Abstract

The spins of binary black holes (BBHs) measured from gravitational waves carry notable information of the formation pathways. Here we propose a quantity "dimensionless net spin" ($\chi_{\rm N}$), which is related to the sum of angular momentum of component black holes in the system, to provide a novel perspective to study the origin(s) of BBHs. By performing hierarchical Bayesian inference on $\chi_{\rm N}$, we find strong evidence that the marginal distribution of this quantity can be better fitted by two Gaussian components rather than one: there is a narrow peak at $\chi_{\rm N} \sim 0.15$ and another extended peak at $\chi_{\rm N} \sim 0.47$. We also find that the rapidly spinning systems likely dominate the high-mass end of the population and they evolve with redshift much quicker. These findings bring new challenges to the field binary scenario, and suggest that dynamical process should plays a key role in forming high total mass BBHs.