Using a combination of Hubble Space Telescope and James Webb Space Telescope imaging, a runaway supermassive black hole was recently identified with an inferred velocity of ${954}_{-126}^{+110}\mathrm{km}{\mathrm{s}}^{-1}$, likely ejected from a compact star-forming galaxy at $z\approx 0.96$. Assuming the runaway black hole originated from a gravitational-wave-driven merger of two supermassive black holes (SMBHs), we combine its measured recoil velocity with gravitational-wave recoil predictions from numerical relativity and black-hole perturbation theory to constrain the mass ratio and spin configuration of the progenitor binary that overcame the final-parsec problem and merged $\sim 70\mathrm{Myr}$ ago. We find that the progenitor binary must have been precessing, with a mass ratio $m_1/m_2\lesssim 6$, and that the more massive SMBH likely possessed a high dimensionless spin magnitude ($\sim 0.75$) in order to generate a recoil of this magnitude. Such SMBH mergers could represent an interesting source population for the upcoming Laser Interferometer Space Antenna mission, with characteristic signal-to-noise ratios of order $\gtrsim {10}^3$. Furthermore, the inferred progenitor SMBH properties suggest that the compact galaxy likely originated from a major, gas-rich (“wet”) merger between two galaxies of comparable mass, with a mass ratio $\lesssim 4$.

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• Formation & evolution of stars & galaxies
• Gravitational waves