The Imprint of Superradiance on Hierarchical Black Hole Mergers
Ultralight bosons are a proposed solution to outstanding problems in cosmology and particle physics: they provide a dark-matter candidate while potentially explaining the strong charge-parity problem. If they exist, ultralight bosons can interact with black holes through the superradiant instability. In this work we explore the consequences of this instability on the evolution of hierarchical black holes within dense stellar clusters. By reducing the spin of individual black holes, superradiance reduces the recoil velocity of merging binary black holes, which, in turn, increases the retention fraction of hierarchical merger remnants. We show that the existence of ultralight bosons with mass 2 × 10⁻¹⁴ ≲ μ/eV ≲ 2 × 10⁻¹³ would lead to an increased rate of hierarchical black hole mergers in nuclear star clusters. An ultralight boson in this energy range would result in up to ≈60% more present-day nuclear star clusters supporting hierarchical growth. The presence of an ultralight boson can also double the rate of intermediate-mass black hole mergers to ≈0.08 Gpc⁻³ yr⁻¹ in the local universe. These results imply that a select range of ultralight boson masses can have far-reaching consequences for the population of black holes in dense stellar environments. Future studies into black hole cluster populations and the spin distribution of hierarchically formed black holes will test this scenario.
© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 July 22; revised 2022 April 9; accepted 2022 April 11; published 2022 May 26. We thank Richard Brito for insightful discussions about black hole superradiance. We also thank Susan Scott and Karl Wette for helpful discussions about ultralight boson cloud detection prospects. This work is supported through Australian Research Council (ARC) Centre of Excellence CE170100004. E.P. acknowledges the support of ARC CE170100004's COVID-19 support fund. P.D.L. is supported through ARC Future Fellowship FT160100112, and ARC Discovery Project DP180103155. K.K. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-2001751. This research has made use of data, software and/or web tools obtained from the Gravitational Wave Open Science Center (https://www.gw-openscience.org), a service of LIGO Laboratory, the LIGO Scientific Collaboration and the Virgo Collaboration. Computing was performed on LIGO Laboratory computing cluster at California Institute of Technology.
Submitted - 2107.11730.pdf
Published - Payne_2022_ApJ_931_79.pdf