CRA Seminar (Dr. Nicholas Stone, Columbia University)

Title: Dynamical Assembly of Black Hole Binaries in Dense Star Systems

Abstract: The discovery of merging black hole (BH) binaries by the LIGO and Virgo experiments has initiated the era of gravitational wave (GW) astrophysics. Detailed comparisons between theoretical and observed waveforms have already tested classical general relativity, and placed stringent constraints on alternative theories of gravity. However, the zeroth-order astrophysical question implied by these GW detections remains unanswered: what is the astrophysical process, and what is the astronomical environment, responsible for producing the bulk of observed BH binary inspirals? A number of different formation pathways have been proposed, and can be classified as either “isolated binary star evolution” or “dynamical assembly.” After broadly overviewing these two formation channels, I will focus on my recent theoretical work to better understand dynamical assembly processes in dense stellar clusters or disks. The best-studied such process is binary-single scatterings in dense star clusters, where repeated three-body interactions create metastable triples that lead to partner swaps and the hardening of binary orbits. The chaotic evolution of these three-body systems is usually studied numerically, but I will present a new analytic formalism that employs the ergodic hypothesis to derive general, closed form statistical solutions for the non-hierarchical three-body problem. This statistical mechanics formalism appears to give reasonable agreement with ensembles of numerical scattering experiments, and I will discuss applications to GW source formation. I will also briefly introduce a novel BH binary formation scenario, the “AGN channel” my collaborators and I have recently proposed. In this scenario, hydrodynamic torques in the gas disks of active galactic nuclei may accelerate the inspiral of wide BH binaries, or pair up singleton BHs into tight orbits. Given ~100 detections, the fractional contribution of this scenario to the total population of GW sources can be deduced from LIGO sky localization volumes alone.