Black Hole Genealogy: Identifying Hierarchical Mergers with Gravitational Waves

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 firstgeneration merger, having a 4% probability of being a hierarchical merger assuming a 5×10 5 M e globular cluster mass. Using our model, we find that 99% of first-generation black holes in coalescing binaries have masses below 44 M e , and the fraction of binaries with near-zero component spins is less than 0.16 (90% probability). Upcoming observations will determine if hierarchical mergers are a common source of gravitational waves.

Constraints from Gravitational-wave Detections of Binary Black Hole Mergers on the 12C(α, γ)16O Rate

“…12 16 ( ) rate. It may also be possible to fit the maximum black hole mass below the gap assuming the population contains both isolated binaries and hierarchical mergers without needing to subtract the hierarchical mergers (Kimball et al 2020) Also, if a channel that produces black holes in the mass gap is rare, we may still be able to determine the location of the mass gap for the more dominant channel.…”

Bayesian analysis of LIGO-Virgo mergers: Primordial versus astrophysical black hole populations

“…[32,68] for recent Bayesian analyses using spin in the PBH context and Ref. [69] for a Bayesian study including a zero-spin black hole population, which could be considered a simplified proxy for a PBH model.…”

Black Hole Mergers from Hierarchical Triples in Dense Star Clusters

“…However, recent works by Rodriguez et al (2020) and Gerosa et al (2020) have shown that the relative fraction of low-mass-ratio merger events can be enhanced by second-and third-generation mergers in dynamical environments, producing GW190412-like events as a result, though analysis by Kimball et al (2020) casts doubt on the hierarchical merger scenario for GW190412. Due to the efficiency of mass segregation, the remnant of a previous merger is expected to be twice as massive as the 1G BHs in the cluster (see also Samsing & Hotokezaka 2020).…”