Abstract:
The formation of merging binary black holes (BBHs) form remains a key unresolved issue in astrophysics, despite nearly 100 detections by the LIGO-Virgo-KAGRA collaboration. Detectable eccentricity offers one of the most promising ways to distinguish different formation channels. However, detecting a sufficient number of eccentric mergers to reliably carry out such a task is expected to be feasible only with third-generation GW detectors, such as the Einstein Telescope or Cosmic Explorer. As these instruments will detect BBH mergers up to redshift z~6, it is critical to understand how the eccentricity distribution evolves with redshift.
We predict the evolution of eccentricity distributions over redshift for merging BBHs from two key channels: the globular cluster (GC) channel and the hierarchical triple channel, where three-body dynamics induce high eccentricities in the inner binary. Our population synthesis method shows that mergers from the GC channel dominate in the local universe (z~0) by an order of magnitude, in broad agreement with previous studies. However, if we focus only on mergers that have detectable eccentricity with third generation detectors (e >10^-4 - 10^-3 at 10 Hz), this picture considerably changes: at z~0, 40% of eccentric mergers arise from hierarchical triples, and this fraction rises to 70% at z~2-3. Therefore, high-redshift eccentric mergers may be dominated by field triples, challenging the view that such mergers primarily occur in dense environments. We also explore the impact of uncertainties in GC and stellar evolution and find that the relative contribution of eccentric mergers from hierarchical triple remains at least ~30-40%. Finally, I will show that the merger rate density and eccentricity distribution of GW sources from the GC channel does not evolve with redshift significantly. This implies that that GW sources formed in GCs could potentially be used to constrain the GC formation history as well as initial parameters of GC.
