Implications of Eccentric Observations on Binary Black Hole Formation Channels

TL;DR

This paper explores how observing eccentric binary black hole mergers with gravitational-wave detectors can reveal the dominant formation channels, especially distinguishing between dynamical cluster origins and isolated evolution, by analyzing selection effects and observational constraints.

Contribution

It demonstrates how the detection or absence of eccentric mergers constrains the contribution of dense star clusters to the overall binary black hole population.

Findings

No eccentric detections after 150 observations suggests clusters are not the main source.

Detection of at least one eccentric merger implies clusters contribute at least 14%.

Tighter constraints on cluster contribution as more eccentric mergers are observed.

Abstract

Orbital eccentricity is one of the most robust discriminators for distinguishing between dynamical and isolated formation scenarios of binary black hole mergers using gravitational-wave observatories such as LIGO and Virgo. Using state-of-the-art cluster models, we show how selection effects impact the detectable distribution of eccentric mergers from clusters. We show that the observation (or lack thereof) of eccentric binary black hole mergers can significantly constrain the fraction of detectable systems that originate from dynamical environments, such as dense star clusters. After roughly 150 observations, observing no eccentric binary signals would indicate that clusters cannot make up the majority of the merging binary black hole population in the local universe (95% credibility). However, if dense star clusters dominate the rate of eccentric mergers and a single system is confirmed to be measurably eccentric in the first and second gravitational-wave transient catalogs, clusters must account for at least 14% of detectable binary black hole mergers. The constraints on the fraction of detectable systems from dense star clusters become significantly tighter as the number of eccentric observations grows and will be constrained to within 0.5 dex once 10 eccentric binary black holes are observed.