LIGO Document P2000131-v3

Black hole genealogy: Identifying hierarchical mergers with gravitational waves from binary black holes

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P - Publications
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03 Apr 2020, 09:44
In dense stellar environments, the merger products of binary black hole coalescences may proceed through additional mergers with other black holes. These hierarchical mergers are predicted to have higher masses and characteristic spins of $\sim 0.7$.
However, since the birth properties of black holes are currently uncertain, it is difficult to distinguish systems where components have already undergone prior mergers.
We outline a inference scheme that uses a set of gravitational-wave observations to reconstruct the binary black hole mass and spin spectrum of a population containing hierarchical merger events.
This inference uses phenomenological models that capture the properties of merging binary black holes from simulations of dense stellar environments, and introduces a zero-spin subpopulation.
We apply our population inference to binary black holes from LIGO and Virgo's first two observing runs, finding that this catalog is consistent with having no hierarchical mergers.
GW170729, the most massive system in this catalog, is the most compatible with having a hierarchical merger origin, with an even odds ratio of $\sim 0.99$ for the binary's heavier black hole having formed in a merger. Using our model, we find that $99\%$ of first-generation black holes in coalescing binaries have masses below $44$ $M_{\odot}$, and the fraction of binaries with near-zero spin is $0.64^{+0.27}_{-0.42}$.
Further observations of binary black holes will improve constraints on the black hole populations residing in dense stellar environments.
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