On the Likely Dynamical Origin of GW191109 and of Binary Black Hole Mergers with Negative Effective Spin

TL;DR

This paper investigates the origin of GW191109 and similar binary black hole mergers with negative effective spin, concluding that a dynamical formation in dense star clusters is the most plausible explanation for their observed properties.

Contribution

The study demonstrates that binary black holes with negative effective spin and high masses are more likely formed through dynamical processes in dense star clusters than isolated binary evolution.

Findings

GW191109's properties are consistent with a dynamical formation scenario.

Isolated binary evolution unlikely explains the spin-orbit misalignment.

Dynamical assembly in star clusters can reproduce the observed features and rates.

Abstract

With the growing number of binary black hole (BBH) mergers detected by LIGO/Virgo/KAGRA, several systems have become difficult to explain via isolated binary evolution, having components in the pair-instability mass gap, high orbital eccentricities, and/or spin-orbit misalignment. Here, we focus on GW191109\_010717, a BBH merger with component masses of $65^{+11}_{-11}$ and $47^{+15}_{-13}$ $\rm M_{\odot}$, and effective spin $-0.29^{+0.42}_{-0.31}$, which could imply a spin-orbit misalignment of more than $\pi/2$ radians for at least one of its components. Besides its component masses being in the pair-instability mass gap, we show that isolated binary evolution is unlikely to reproduce the proposed spin-orbit misalignment of GW191109 with high confidence. On the other hand, we demonstrate that BBHs dynamically assembled in dense star clusters would naturally reproduce the spin-orbit misalignment and the masses of GW191109, and the rates of GW191109-like events, if at least one of the components were to be a second-generation BH. Finally, we generalize our results to all the events with a measured negative effective spin, arguing that GW200225 also has a likely dynamical origin.