Building better spin models for merging binary black holes: Evidence for non-spinning and rapidly spinning nearly aligned sub-populations

TL;DR

This paper analyzes black hole spin distributions in merging binaries using gravitational wave data, revealing two sub-populations: one with negligible spins and another with aligned, moderately spinning black holes, supporting the field formation scenario.

Contribution

It introduces a revised spin model allowing for negligible spins and reanalyzes GWTC-2 data, confirming the existence of two distinct black hole spin sub-populations.

Findings

Approximately 29-75% of binaries have negligible spins.

The remaining binaries have spins around 0.45 with aligned vectors.

Results support the field formation scenario for black hole binaries.

Abstract

Recent work paints a conflicting portrait of the distribution of black hole spins in merging binaries measured with gravitational waves. Some analyses find that a significant fraction of merging binaries contain at least one black hole with a spin tilt $>90^\circ$ with respect to the orbital angular momentum vector, which has been interpreted as a signature for dynamical assembly. Other analyses find the data are consistent with a bimodal population in which some binaries contain black holes with negligible spin while the rest contain black holes with spin vectors preferentially aligned with the orbital angular momentum vector. In this work, we scrutinize models for the distribution of black hole spins to pinpoint possible failure modes in which the model yields a faulty conclusion. We reanalyze data from the second LIGO--Virgo gravitational-wave transient catalog (GWTC-2) using a revised spin model, which allows for a sub-population of black holes with negligible spins. In agreement with recent results by Roulet et al., we show that the GWTC-2 detections are consistent with two distinct sub-populations. We estimate that $29-75\%$ (90\% credible interval) of merging binaries contain black holes with negligible spin $\chi \approx 0$. The remaining binaries are part of a second sub-population in which the spin vectors are preferentially (but not exactly) aligned to the orbital angular momentum. The black holes in this second sub-population are characterized by spins of $\chi\sim0.45$. We suggest that the inferred spin distribution is consistent with the hypothesis that all merging binaries form via the field formation scenario.