Building Better Spin Models for Merging Binary Black Holes: Evidence for Nonspinning and Rapidly Spinning Nearly Aligned Subpopulations
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° with respect to the orbital angular momentum vector, which has been interpreted as a signature for dynamical assembly. Other analyses find that 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 subpopulation 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 subpopulations. We estimate that 69%–90% (90% credible interval) of merging binaries contain black holes with negligible spin χ ≈ 0. The remaining binaries are part of a second subpopulation in which the spin vectors are preferentially (but not exactly) aligned to the orbital angular momentum. The black holes in this second subpopulation are characterized by spins of χ ∼ 0.5. We suggest that the inferred spin distribution is consistent with the hypothesis that all merging binaries form via the field formation scenario.
© 2021. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 September 6; revised 2021 October 7; accepted 2021 October 11; published 2021 October 29. We thank Tom Callister for his helpful comments during the preparation of this manuscript. Several authors are supported by the Australian Research Council (ARC) Centre of Excellence for Gravitational-wave Discovery (OzGrav), project number CE170100004. I.M. is the recipient of the ARC Future Fellowship FT190100574. The authors are grateful for computational resources provided by the LIGO Laboratory and supported by National Science Foundation Grants PHY-0757058 and PHY-0823459. This is document LIGO-P2100318. Software: GWPopulation Talbot et al. (2019), Bilby Ashton et al. (2019); Romero-Shaw et al. (2020), dynesty Speagle (2020).
Accepted Version - 2109.02424.pdf
Published - Galaudage_2021_ApJL_921_L15.pdf