Progenitor of the recoiling super-massive black hole RBH-1 identified using HST/JWST imaging

TL;DR

This study combines HST and JWST imaging to analyze a recoiling supermassive black hole, constraining its progenitor's properties and implications for galaxy mergers and future gravitational wave observations.

Contribution

It provides the first constraints on the progenitor SMBH binary parameters based on recoil velocity measurements and gravitational-wave recoil models.

Findings

Progenitor SMBH binary was precessing with a mass ratio less than 6.

The more massive SMBH had a high spin (~0.75) to produce the observed recoil.

GX galaxy likely formed from a major, gas-rich merger of similar-mass galaxies.

Abstract

Using a combination of \textit{Hubble Space Telescope} and \textit{James Webb Space Telescope} imaging, a runaway supermassive black hole (RBH-1) was recently identified with an inferred velocity of $954^{+110}_{-126}\,\mathrm{km\,s^{-1}}$, likely ejected from a compact star-forming galaxy (denoted as GX) at $z \approx 0.96$. Assuming the runaway black hole was the outcome of the gravitational-wave-driven merger of two black holes, we use its measured runaway velocity together with gravitational-wave recoil predictions from numerical relativity and black hole perturbation theory to constrain the mass ratio and spin configuration of the progenitor SMBHs that overcame the final-parsec problem and merged $\sim 70$~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 must have possessed a high spin (dimensionless spin magnitude $\sim 0.75$) in order to generate a recoil of this magnitude. This has important astrophysical implications as similar SMBH mergers can be an interesting source population for the upcoming LISA mission with signal-to-noise ratios $\gtrsim$ 1000. Furthermore, the progenitor SMBH properties imply that GX was likely formed through a major, gas-rich (``wet'') merger between two galaxies of comparable mass, with a mass ratio $\lesssim 4$.