Dropping Anchor: Understanding the Populations of Binary Black Holes with Random and Aligned Spin Orientations

TL;DR

This paper introduces a model to distinguish between dynamical and field binary black hole populations based on spin orientations, analyzing GWTC-3 data to explore their properties and evolutionary origins.

Contribution

The authors develop a simple population model linking spin orientations to binary formation channels and apply it to gravitational-wave data to investigate differences in properties.

Findings

No significant difference in mass ratio, spin magnitude, or redshift distributions between populations.

Both populations cannot both favor equal mass ratios strongly.

Current data insufficient to distinguish differences in spin magnitude or redshift distributions.

Abstract

The relative spin orientations of black holes (BHs) in binaries encode their evolutionary history: BHs assembled dynamically should have isotropically distributed spins, while spins of the BHs originating in the field should be aligned with the orbital angular momentum. In this article, we introduce a simple population model for these dynamical and field binaries that uses spin orientations as an anchor to disentangle these two evolutionary channels. We then analyze binary BH mergers in the Third Gravitational-Wave Transient Catalog (GWTC-3) and ask whether BHs from the isotropic-spin population possess different distributions of mass ratio, spin magnitudes, or redshifts from the preferentially-aligned-spin population. We find no compelling evidence that binary BHs in GWTC-3 have different source-property distributions depending on their spin alignment, but we do find that the dynamical and field channels cannot both have mass-ratio distributions that strongly favor equal masses. We give an example of how this can be used to provide insights into the various processes that drive these BHs to merge. We also find that the current detections are insufficient in extracting differences in spin magnitude or redshift distributions of isotropic and aligned spin populations.