Joint constraints on the field-cluster mixing fraction, common envelope efficiency, and globular cluster radii from a population of binary hole mergers via deep learning

Abstract

The recent release of the second Gravitational-Wave Transient Catalog (GWTC-2) has increased significantly the number of known GW events, enabling unprecedented constraints on formation models of compact binaries. One pressing question is to understand the fraction of binaries originating from different formation channels, such as isolated field formation versus dynamical formation in dense stellar clusters. In this paper, we combine the COSMIC binary population synthesis suite and the CMC code for globular cluster evolution to create a mixture model for black hole binary formation under both formation scenarios. For the first time, these code bodies are combined self-consistently, with CMC itself employing COSMIC to track stellar evolution. We then use a deep-learning enhanced hierarchical Bayesian analysis to continuously sample over and constrain the common envelope efficiency α assumed in COSMIC, the initial cluster virial radius r v adopted in CMC, and the intrinsic mixture fraction f between each channel. Under specific assumptions about other uncertain aspects of isolated binary and globular cluster evolution, we report the median and 90% confidence interval of three physical parameters for the intrinsic population ðf; α; r v Þ ¼ ð0.20 þ0.32 −0.18 ; 2.26 þ2.65 −1.84 ; 2.71 þ0.83 −1.17 Þ. This simultaneous constraint agrees with observed properties of globular clusters in the Milky Way and is an important first step in the pathway toward learning the astrophysics of compact binary formation through GW observations.

Mass and Rate of Hierarchical Black Hole Mergers in Young, Globular and Nuclear Star Clusters

“…For example, the posterior distributions of the mixing fraction of the NSC, YSC and Field channels peak at values significantly larger than zero in the A5F05, LONG_DELAY, SHORT_DELAY, HIGH_SPIN, BROAD_VESC, and NARROW_VESC models. This result is in agreement with previous work [189][190][191][192].…”

Rates of compact object coalescences

“…On the one hand, increasingly complex models of binary evolution and dynamics are sensitive to a broad range of uncertain assumptions about initial conditions, stellar evolution, mass loss, mass transfer, supernova kicks, and the cosmic star formation history of the Universe. On the other hand, there is growing evidence that observations of the rates and properties of compact-object mergers, coupled with other observational constraints, will make it possible to resolve these uncertainties and come away with a deep understanding of the physics of compact binary formation (e.g., Mandel and O'Shaughnessy 2010;Stevenson et al 2015;Barrett et al 2018;Wong et al 2021).…”

Evidence for Hierarchical Black Hole Mergers in the Second LIGO–Virgo Gravitational Wave Catalog

“…While illustrative, this is unrealistic as (i) the observed BBH population may come from a mixture of formation channels including isolated field evolution, and (ii) dynamically formed binaries may occur in a wide range cluster types ranging from young open clusters to nuclear star clusters. An excess of events with aligned spin in GWTC-2 suggests that at least some binaries are assembled in the field (Abbott et al 2021b), and comparisons of observations with model predictions indicate that a mix of formation channels is probable (Bouffanais et al 2021;Wong et al 2021;Zevin et al 2021). The potential for multiple formation channels could be accounted for by including an additional mixture model for dynamically formed binaries versus those formed in isolation (Kimball et al 2020b).…”