# Distinguishing Spin-Aligned and Isotropic Black Hole Populations With Gravitational Waves

Research output: Contribution to journalArticlepeer-review

## Authors

• Simon Stevenson
• Ben Farr
• M Coleman Miller

## External organisations

• University of Maryland, College Park, Maryland 20742 USA

## Abstract

The first direct detections of gravitational waves from merging binary black holes open a unique window into the binary black hole formation environment. One promising environmental signature is the angular distribution of the black hole spins; systems formed through dynamical interactions among already-compact objects are expected to have isotropic spin orientations whereas binaries formed from pairs of stars born together are more likely to have spins preferentially aligned with the binary orbital angular momentum. We consider existing gravitational wave measurements of the binary effective spin, the best-measured combination of spin parameters, in the four likely binary black hole detections GW150914, LVT151012, GW151226, and GW170104. If binary black hole spin magnitudes extend to high values we show that the data exhibit a $2.4\sigma$ ($0.015$ odds ratio) preference for an isotropic angular distribution over an aligned one. By considering the effect of 10 additional detections, we show that such an augmented data set would enable in most cases a preference stronger than $5\sigma$ ($2.9 \times 10^{-7}$ odds ratio). The existing preference for either an isotropic spin distribution or low spin magnitudes for the observed systems will be confirmed (or overturned) confidently in the near future.

## Bibliographic note

32 pages, 9 figures, code and document at https://github.com/farr/AlignedVersusIsoSpin/ ; updated to use custom LaTeX class to include figures in PDF

## Details

Original language English 426-429 Nature 548 23 Aug 2017 E-pub ahead of print - 23 Aug 2017

## Keywords

• astro-ph.HE, gr-qc