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 firstgeneration black holes are uncertain, it is difficult to know if any given merger is firstgeneration 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 lowspin black holes, we include a zerospin 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_{⊙} globular cluster mass. Using our model, we find that 99% of firstgeneration black holes in coalescing binaries have masses below $44$ M_{⊙}, and the fraction of binaries with nearzero component spins is less than $0.16$ (90% probability). Upcoming observations will determine if hierarchical mergers are a common source of gravitational waves.
 Publication:

The Astrophysical Journal
 Pub Date:
 September 2020
 DOI:
 10.3847/15384357/aba518
 arXiv:
 arXiv:2005.00023
 Bibcode:
 2020ApJ...900..177K
 Keywords:

 Gravitational wave sources;
 Gravitational wave astronomy;
 Astrophysical black holes;
 Hierarchical models;
 677;
 675;
 98;
 1925;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology
 EPrint:
 Appendix B added to include GW190412 in the analysis