Signs of eccentricity in two gravitational-wave signals may indicate a sub-population of dynamically assembled binary black holes

TL;DR

This paper analyzes gravitational-wave signals from binary black hole mergers to detect orbital eccentricity, providing evidence for a sub-population formed dynamically in dense stellar environments, which impacts our understanding of black hole formation channels.

Contribution

The study presents the first significant measurement of eccentricity in a binary black hole merger, expanding the analysis to 36 events and highlighting the potential prevalence of dynamically assembled binaries.

Findings

GW190620A shows evidence of non-zero eccentricity.

Approximately 27% of LIGO-Virgo binary black holes may be dynamically formed.

Waveform systematics currently limit definitive eccentricity measurements.

Abstract

The orbital eccentricity of a merging binary black hole leaves an imprint on the associated gravitational-wave signal that can reveal whether the binary formed in isolation or in a dynamical environment, such as the core of a dense star cluster. We present measurements of the eccentricity of 26 binary black hole mergers in the second LIGO--Virgo gravitational-wave transient catalog, updating the total number of binary black holes analysed for orbital eccentricity to 36. Using the \texttt{SEOBNRE} waveform, we find the data for GW190620A is poorly explained by the zero-eccentricity hypothesis (frequentist $p$-value $\lesssim 0.1\%$). Using a log-uniform prior on eccentricity, the eccentricity at $\unit[10]{Hz}$ for GW190620A is constrained to $e_{10}\geq0.05$ ($0.1$) at $74\%$ ($65\%$) credibility. With this log-uniform prior, we obtain a $90\%$ credible lower eccentricity limit of $0.001$, while assuming a uniform prior leads the data to prefer $e_{10} \geq 0.11$ at $90\%$ credibility. This is the second measurement of a binary black hole system with statistical support for non-zero eccentricity; the intermediate-mass black hole merger GW190521 was the first. Interpretation of these two events is currently complicated by waveform systematics; we are unable to simultaneously model the effects of relativistic precession and eccentricity. However, if these two events are, in fact, eccentric mergers, then there are potentially many more dynamically assembled mergers in the LIGO--Virgo catalog without measurable eccentricity; $\gtrsim 27\%$ of the observed LIGO--Virgo binaries may have been assembled dynamically in dense stellar environments ($95\%$ credibility).