Dynamical pairing in gravitational wave populations

Abstract

Binary black holes formed dynamically in star clusters are often associated with nearly equal-mass pairing. We show that this expectation is incomplete. The more general prediction of dynamical assembly is an ordered-pairing relation in which the two black holes are drawn independently from a common one--body mass spectrum and then ordered by mass. Once the primary-mass distribution is known, this relation fixes the conditional companion distribution $p(m_2\mid m_1)$. We validate this picture with cluster simulations. In these models, the ordered-draw relation reproduces the morphology of the mass-pairing distribution, including fixed-$m_2$ ridges that are inherited from the one--body mass spectrum: features at nearly constant secondary mass that extend over a range of primary masses and therefore correspond to decreasing $q=m_2/m_1$ as $m_1$ increases. Thus, the geometry of the mass distribution itself becomes a formation-channel diagnostic. Dynamical pairing maps peaks of the primary-mass spectrum into fixed-$m_2$ ridges with predictable amplitude, whereas isolated binary evolution more naturally produces structures at fixed $q$. Applying this test to a non-parametric reconstruction of the gravitational-wave population, we find a tentative enhancement at $m_2\simeq 30M_{\odot }$ for $m_1\gtrsim 40M_{\odot }$ that is broadly compatible with ordered pairing. Finally, we show that dynamical formation naturally explains the rapid decline of the secondary-mass distribution above $\sim 30M_{\odot }$ recently reported by the LIGO--Virgo--KAGRA Collaboration.

arXiv comment20 pages, 11 figures. Comments are welcomed