Detailed properties of gravitational-wave mergers from flyby perturbations of wide binary black holes in the field

TL;DR

This study models how flyby perturbations of wide binary black holes in the field can produce gravitational wave sources, providing detailed predictions on their rates and properties based on population synthesis.

Contribution

It introduces detailed population synthesis models to predict GW source properties and merger rates from flyby-perturbed wide binary black holes, considering various natal-kick and metallicity scenarios.

Findings

Merger rates of 1-20 Gpc$^{-3}$ yr$^{-1}$ consistent with observations.

Produced GW sources are circularized in the LIGO band.

GW sources favor high velocity dispersion host galaxies.

Abstract

Wide black hole binaries (wide-BBHs; $\geqslant 10^3$ AU) in the field can be perturbed by random stellar flybys that excite their eccentricities. Once a wide binary is driven to a sufficiently small pericenter approach, gravitational wave (GW) emission becomes significant, and the binary inspirals and merges. In our previous study, using simplified models for wide-BBHs, we found that successive flybys lead to significant merger fractions of wide-BBHs in less than Hubble time, making the flyby perturbation mechanism a relevant contributor to the production rate of GW-sources. However, the exact rates and detailed properties of the resulting GW sources depend on the wide binary progenitors. In this paper we use detailed population synthesis models for the initial wide-BBH population, considering several populations corresponding to different natal-kick models and metallicities, and then follow the wide-BBHs evolution due to flyby perturbations and GW-emission. We show that the cumulative effect of flybys is conductive for the production of GW sources in non-negligible rates of $1-20$ Gpc$^{-3}$ yr$^{-1}$, which are sensitive to the natal kicks model. Such rates are relevant to the observationally inferred rate. Our models, now derived from detailed population of binaries, provide the detailed properties of the produced GW-sources, including mass-functions and delay times. The GW mergers are circularized when enter the aLIGO band; have a preference for high velocity dispersion host galaxies (in particular ellipticals); have a relatively uniform delay-time distribution; and likely have mildly correlated (less than isolated evolution channels and more than dynamical channels) prograde spin-spin and spin-orbits.