Implications of Eccentric Observations on Binary Black Hole Formation Channels

Michael Zevin,Isobel M Romero-Shaw,Paul D Lasky,Eric Thrane,Kyle Kremer

doi:10.3847/2041-8213/ac32dc

Abstract

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.

Highlights

In the past few years, the dramatic increase in compact binary mergers observed by gravitational-wave (GW) detectors has fueled immense interest and debate regarding compact binary formation pathways, for binary black hole (BBH) systems
Over a dozen potential formation scenarios for the BBH mergers observed by the LIGO–Virgo detector network (Aasi et al 2015; Acernese et al 2015) have been proposed, including isolated massive-star binary progenitors (e.g., Bethe & Brown 1998; Dominik et al 2012; Belczynski et al 2016; Bavera et al 2021), assembly in dynamical environments (e.g., Portegies Zwart & McMillan 2000; O’Leary et al 2006; Downing et al 2010; Rodriguez et al 2016; Banerjee 2017; Di Carlo et al 2019), gas-driven assembly and orbital evolution (e.g., McKernan et al 2014; Bartos et al 2017; Stone et al 2017), and primordial origins (e.g., Bird et al 2016; Sasaki et al 2018; Clesse & Garcia-Bellido 2020; Franciolini et al 2021)
Operating under the assumption that Necc out of the Nobs = 46 BBH observations in GWTC-2 are measurably eccentric, we focus on the constraints that can be placed on βc given the presence of eccentric observations

Summary

Introduction

In the past few years, the dramatic increase in compact binary mergers observed by gravitational-wave (GW) detectors has fueled immense interest and debate regarding compact binary formation pathways, for binary black hole (BBH) systems. Though population-based studies offer insights into the broad features of BBH formation, the most efficient means of constraining formation scenarios is to identify features of BBH systems unique to particular channels One such key feature is orbital eccentricity. The only means of producing measurably eccentric BBH mergers in the LIGO–Virgo band that does not require a high degree of fine-tuning is through strong gravitational encounters in dynamical environments (e.g., O’Leary et al 2009; Kocsis & Levin 2012; Samsing et al 2014; Samsing et al 2018; Samsing 2018; Gondán et al 2018; Rodriguez et al 2018b; Takatsy et al 2019; Zevin et al 2019; Rasskazov & Kocsis 2019; Gröbner et al 2020; Samsing et al 2020b; Gondán & Kocsis 2021; Tagawa et al 2021) or channels that can pump eccentricity into inspiraling binaries, such as the secular evolution of hierarchical systems We use a flat ΛCDM cosmology with Planck 2015 cosmological parameters (Ade et al 2016) throughout this work

Cluster Models

Selection Effects and Measurability of Eccentric Sources

Findings

Discussion and Conclusions