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

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 • 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 70 − 90% (90% credible interval) of merging binaries contain black holes with negligible spin χ ≈ 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 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.1. INTRODUCTION Gravitational waves from merging binaries encode information about the mass and spin of the component black holes and/or neutron stars. These properties, in turn, provides clues as to how the binary formed. Two scenarios are frequently invoked to explain merging binary black holes: field and dynamical (see Mandel & Farmer 2018; Mapelli 2021; Mandel & Broekgaarden 2021 for reviews). In the field scenario, binary black hole (BBH) systems are formed from isolated stellar binaries. In the dynamical scenario, BBH systems are assembled through interactions in dense stellar environ-

Understanding how fast black holes spin by analysing data from the second gravitational-wave catalogue

“…Secondly, we show that our results agree with those presented in Roulet et al (2021) and Galaudage et al (2021) and show that there is evidence that the black hole population is consistent with two spin subpopulations, which, when combined, gives an overall preference for the distribution in Abbott et al (2021b). We highlight that the majority of events (∼ 80%) prefer a spin distribution with extremely low spin magnitudes while several show preference for larger spins.…”

“…This hints at possible sub-populations in the preferred spin distribution: one with negligible spins (EL) and one with larger spins (L). This is the same conclusion found by Roulet et al (2021) and Galaudage et al (2021). We therefore investigate these possible spin sub-populations by calculating odds ratios between models with negligible spin (ELI) and models with larger spins both nearly aligned with the orbital angular momentum (LA) and spins isotropically distributed (LI).…”

“…By assuming a population model where the spin magnitude of each black hole is described by a beta function (Wysocki et al 2019) and the orientation by a model allowing for both isotropic and aligned spins (Talbot & Thrane 2017), it was shown that the most likely spin distribution has a peak in the spin magnitude at ∼ 0.2, with preference for primarily aligned spins (although there is non-vanishing support for angles > 90 • indicating the presence of misaligned component spins). Roulet et al (2021) and Galaudage et al (2021) later challenged this point of view, showing that all binaries observed to date are consistent with two spin populations: one with negligible black hole spin and a second with spins preferentially aligned with the orbital angular momentum. Galaudage et al (2021) estimated that 70 -90% of merging binaries contain black holes with negligible spin and the black holes in the second sub-population have spins ∼ 0.5 with orientations preferentially (but not exactly) aligned to the orbital angular momentum.…”

“…Roulet et al (2021) and Galaudage et al (2021) later challenged this point of view, showing that all binaries observed to date are consistent with two spin populations: one with negligible black hole spin and a second with spins preferentially aligned with the orbital angular momentum. Galaudage et al (2021) estimated that 70 -90% of merging binaries contain black holes with negligible spin and the black holes in the second sub-population have spins ∼ 0.5 with orientations preferentially (but not exactly) aligned to the orbital angular momentum. Callister et al (2021) also searched for correlations within the black hole spin distribution and found that the binaries mass ratio is correlated with the black hole spin at 98.7% credibility with more unequal mass binaries exhibiting systematically larger black hole spin.…”

Exploring Features in the Binary Black Hole Population

“…Further observations have expanded on these features, indicating that lighter binaries contribute significantly to the total merger rate (Abbott et al 2019a(Abbott et al , 2021c, black holes in heavier binaries tend to have larger spin magnitude Galaudage et al 2021;Hoy et al 2021), and as reported in this article the early results indicate the merger rate evolution of peaks in the mass distribution is disparate.…”