Abstract
Abstract In dense stellar environments, the merger products of binary black hole mergers may undergo additional mergers. These hierarchical mergers are naturally expected to have higher masses than the first generation of black holes made from stars. The components of hierarchical mergers are expected to have significant characteristic spins, imprinted by the orbital angular momentum of the previous mergers. However, since the population properties of first-generation black holes are uncertain, it is difficult to know if any given merger is first-generation or hierarchical. We use observations of gravitational waves to reconstruct the binary black hole mass and spin spectrum of a population including the possibility of hierarchical mergers. We employ a phenomenological model that captures the properties of merging binary black holes from simulations of globular clusters. Inspired by recent work on the formation of low-spin black holes, we include a zero-spin subpopulation. We analyze binary black holes from LIGO and Virgo’s first two observing runs, and find that this catalog is consistent with having no hierarchical mergers. We find that the most massive system in this catalog, GW170729, is mostly likely a first-generation merger, having a 4% probability of being a hierarchical merger assuming a 5 × 105 M ⊙ globular cluster mass. Using our model, we find that 99% of first-generation black holes in coalescing binaries have masses below M ⊙, and the fraction of binaries with near-zero component spins is less than (90% probability). Upcoming observations will determine if hierarchical mergers are a common source of gravitational waves.
Highlights
The gravitational-wave (GW) observations of LIGO (Aasi et al 2015) and Virgo (Acernese et al 2015) have revealed a population of stellar-mass binary black holes (Abbott et al 2016a, 2019a, 2020a)
We find that observations are consistent with all binaries being 1G+1G (Chatziioannou et al 2019; Kimball et al 2020; Yang et al 2019); if we include the possibility that some 1G black holes are born with near-zero spins (Qin et al 2018; Fuller & Ma 2019; Belczynski et al 2020), we find a small probability of GW170729 containing a 2G black hole using our models for globular clusters
The model we develop in this study approximates the detectable population of merging binary black holes from globular clusters, and is designed to capture the main features of binaries formed through hierarchical mergers
Summary
The gravitational-wave (GW) observations of LIGO (Aasi et al 2015) and Virgo (Acernese et al 2015) have revealed a population of stellar-mass binary black holes (Abbott et al 2016a, 2019a, 2020a). In dense stellar environments, such as globular clusters and nuclear star clusters, gravitational encounters of black holes in the cluster core harden the orbits of binary black hole systems, facilitating mergers within the cluster (e.g., Heggie 1975; Banerjee et al 2010; Rodriguez et al 2016a). If these merger products remain in the cluster environment, they can potentially merge again.
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