New insights on binary black hole formation channels after GWTC2: young star clusters versus isolated binaries
Abstract
With the recent release of the Second Gravitational Wave Transient Catalog (GWTC2), which introduced dozens of new detections, we are at a turning point of gravitational wave astronomy, as we are now able to directly infer constraints on the astrophysical population of compact objects. Here, we tackle the burning issue of understanding the origin of binary black hole (BBH) mergers. To this effect, we make use of stateoftheart population synthesis and Nbody simulations, to represent two distinct formation channels: BBHs formed in the field (isolated channel) and in young star clusters (dynamical channel). We then use a Bayesian hierarchical approach to infer the distribution of the mixing fraction f, with f = 0 (f = 1) in the pure dynamical (isolated) channel. We explore the effects of additional hyperparameters of the model, such as the spread in metallicity σ_{Z} and the parameter σ_{sp}, describing the distribution of spin magnitudes. We find that the dynamical model is slightly favoured with a median value of f = 0.26, when σ_{sp} = 0.1 and σ_{Z} = 0.4. Models with higher spin magnitudes tend to strongly favour dynamically formed BBHs (f ≤ 0.1 if σ_{sp} = 0.3). Furthermore, we show that hyperparameters controlling the rates of the model, such as σ_{Z}, have a large impact on the inference of the mixing fraction, which rises from 0.18 to 0.43 when we increase σ_{Z} from 0.2 to 0.6, for a fixed value of σ_{sp} = 0.1. Finally, our current set of observations is better described by a combination of both formation channels, as a pure dynamical scenario is excluded at the $99{{\ \rm per\ cent}}$ credible interval, except when the spin magnitude is high.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 November 2021
 DOI:
 10.1093/mnras/stab2438
 arXiv:
 arXiv:2102.12495
 Bibcode:
 2021MNRAS.507.5224B
 Keywords:

 black hole physics;
 gravitational waves;
 methods: numerical;
 methods: statistical;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 13 pages, 10 figures, 2 tables, published in MNRAS