Structure and Skewness of the Effective Inspiral Spin Distribution of Binary Black Hole Mergers

TL;DR

This paper analyzes the distribution of effective inspiral spins in binary black hole mergers, revealing non-Gaussian features that suggest multiple formation channels and a subpopulation with aligned spins, based on current gravitational wave data.

Contribution

It demonstrates that non-Gaussian features in the effective spin distribution can indicate multiple formation channels and estimates the fraction of aligned-spin subpopulations from observational data.

Findings

Signs of skewness and asymmetry in current catalogs.

At least 12-17% of systems have aligned spins.

Approximately 20% of binaries have negative effective spins.

Abstract

The detection of gravitational waves has brought to light a population of binary black holes that merge within a Hubble time. Multiple formation channels can contribute to this population, making it difficult to definitively associate particular population features with underlying stellar physics. Black hole spins are considered an important discriminator between various channels, but they are less well-measured than masses, making conclusive astrophysical statements using spins difficult thus far. In this paper, we consider the distribution of the effective inspiral spin $\chi_{\rm eff}$ -- a quantity much better measured than individual component spins. We show that non-Gaussian features like skewness, asymmetry about zero, and multimodality can naturally arise in the $\chi_{\rm eff}$ distribution when multiple channels contribute to the population. Searching for such features, we find signs of skewness and asymmetry already in the current catalogs, but no statistically significant signs of bimodality. These features provide robust evidence for the presence of a subpopulation with spins preferentially aligned to the binary's orbital angular momentum; and we conservatively estimate the fraction of this subpopulation to be at least $12 \% - 17\%$ (at $90\%$ credibility). Our models do not find an excess of non-spinning systems and instead find that at least $\sim 20 \%$ of the binaries have some degree of negative $\chi_{\rm eff}$. The data also suggest that, if preferentially aligned mergers form a significant fraction of the population, they must have small spins.