Constraining stellar binary black hole formation scenarios with eLISA eccentricity measurements

TL;DR

eLISA observations of black hole binary eccentricities can distinguish their formation channels, with a few observations identifying those near massive black holes and tens needed to differentiate field from cluster origins.

Contribution

This study demonstrates that eLISA's eccentricity measurements can effectively identify black hole binary formation scenarios using Bayesian model selection.

Findings

A handful of eLISA observations can identify binaries formed near massive black holes.

Several tens of observations are needed to distinguish field from globular cluster formation.

A five-year eLISA mission enhances the ability to constrain formation channels.

Abstract

A space-based interferometer such as eLISA could observe few to few thousands progenitors of black hole binaries (BHBs) similar to those recently detected by Advanced LIGO. Gravitational radiation circularizes the orbit during inspiral, but some BHBs retain a measurable eccentricity at the low frequencies where eLISA is most sensitive. The eccentricity of a BHB carries precious information about its formation channel: BHBs formed in the field, in globular clusters, or close to a massive black hole (MBH) have distinct eccentricity distributions in the eLISA band. We generate mock eLISA observations, folding in measurement errors, and using Bayesian model selection we study whether eLISA measurements can identify the BHB formation channel. We find that a handful of observations would suffice to tell whether BHBs were formed in the gravitational field of a MBH. Conversely, several tens of observations are needed to tell apart field formation from globular cluster formation. A five-year eLISA mission with the longest possible armlength is desirable to shed light on BHB formation scenarios.