Four eccentric mergers increase the evidence that LIGO--Virgo--KAGRA's binary black holes form dynamically

TL;DR

This study analyzes 62 binary black hole merger candidates from LIGO-Virgo-KAGRA data, finding evidence for significant orbital eccentricity in at least four events, which has implications for their formation environments.

Contribution

It provides the first evidence of eccentricity in multiple binary black hole mergers, suggesting dense star clusters as a primary formation channel and outlining future detection prospects.

Findings

At least four events show significant eccentricity support.

Two new eccentric candidates: GW191109 and GW200208_22.

Potential for future discrimination of formation channels with more detections.

Abstract

The growing population of compact binary mergers detected with gravitational waves contains multiple events that are challenging to explain through isolated binary evolution. Such events have higher masses than are expected in isolated binaries, component spin-tilt angles that are misaligned, and/or non-negligible orbital eccentricities. We investigate the orbital eccentricities of 62 binary black hole candidates from the third gravitational-wave transient catalogue of the LIGO-Virgo-KAGRA Collaboration with an aligned-spin, moderate-eccentricity waveform model. Within this framework, we find that at least four of these events show significant support for eccentricity $e_{10} \geq 0.1$ at a gravitational-wave frequency of $10$~Hz ($> 60\%$ credibility, under a log-uniform eccentricity prior that spans the range $10^{-4} < e_{10} < 0.2$). Two of these events are new additions to the population: GW191109 and GW200208\_22. If the four eccentric candidates are truly eccentric, our results suggest that densely-populated star clusters may produce 100\% of the observed mergers. However, it remains likely that other formation environments with higher yields of eccentric mergers -- for example, active galactic nuclei -- also contribute. We estimate that we will be able to confidently distinguish which formation channel dominates the eccentric merger rate after $\gtrsim 80$ detections of events with $e_{10} \geq 0.05$ at LIGO--Virgo sensitivity, with only $\sim 5$ detectably-eccentric events required to distinguish formation channels with third-generation gravitational-wave detectors.