2021ApJ...914L..18D

The inspiral and merger of two black holes produces a remnant black hole with mass and spin determined by the properties of its parent black holes. Using the inferred population properties of component black holes from the first two and a half observing runs of Advanced LIGO and Virgo, we calculate the population properties of the leftover remnant black holes. By integrating their rate of formation over the age of the universe, we estimate the number density of remnant black holes today. Using simple prescriptions for the cosmic star formation rate and black hole inspiral delay times, we determine the number density of this leftover black hole population to be 660_–240⁺⁴⁴⁰Mpc^–3, corresponding to ∼60,000 black hole remnants per Milky Way-equivalent galaxy. The mass spectrum of these remnants starts at ∼10 M_☉ and can be approximated by a decreasing exponential with characteristic length ∼15 M_☉, the final spin distribution is sharply peaked at χ_f ∼ 0.7, and the kick velocities range from tens to thousands of kilometers per second. These kick velocities suggest that globular clusters and nuclear star clusters may retain up to 3_–2⁺³ % and 46_–15⁺¹⁷ % of their remnant black holes, respectively, while young star clusters would only retain a few tenths of a percent. The estimates in this work assume that none of the remnants participate in subsequent hierarchical mergers. If hierarchical mergers occur, the overall number density would drop accordingly and the remnant mass distribution shape would evolve over time. This population of leftover black holes is an inescapable result from gravitational-wave observations of binary black hole mergers.