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

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 talk, I will 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 can use this model to 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 also find that the current detections are insufficient in extracting differences in spin magnitude or redshift distributions of isotropic and aligned spin populations. 

I will give an example of how this can be used to provide insights into the various processes that drive these BHs to merge.