2023ApJ...945L..18P

The remnants of binary black hole mergers can be given recoil kick velocities up to 5000 km s^–1 due to anisotropic emission of gravitational waves. E1821+643 is a recoiling supermassive black hole candidate with spectroscopically offset, broad emission lines, consistent with motion of the black hole at ∼2100 km s^–1 along the line of sight relative to its host galaxy. This suggests a recoil kick of ∼2200 km s^–1. Such a kick is powerful enough to eject E1821+643 from its M_gal ∼ 2 × 10¹²M_⊙ host galaxy. In this work, we address the question: assuming that E1821+643 is a recoiling black hole, what are the likely properties of the progenitor binary that formed E1821+643? Using astrophysically motivated priors, we infer that E1821+643 was likely formed from a binary black hole system with masses of ${m}_{1}\sim {1.9}_{-0.4}^{+0.5}\times {10}^{9}{M}_{\odot }$, ${m}_{2}\sim {8.1}_{-3.2}^{+3.9}\times {10}^{8}{M}_{\odot }$ (90% credible intervals). Given our model, the black holes in this binary were likely to be spinning rapidly with dimensionless spin magnitudes of ${\chi }_{1}={0.87}_{-0.26}^{+0.11}$, ${\chi }_{2}={0.77}_{-0.37}^{+0.19}$. Such a high recoil velocity is impossible for spins aligned to the orbital angular momentum axis. This suggests that the progenitor for E1821+643 merged in hot gas, which is thought to provide an environment where spin alignment from accretion proceeds slowly relative to the merger timescale. We infer that E1821+643, if it is a recoiling black hole, is likely to be a rapidly rotating black hole with a dimensionless spin of χ = 0.92 ± 0.04. A 2.6 × 10⁹M_⊙ _black hole, recoiling from a gas-rich environment at v ∼ 2200 km s^–1, is likely to persist as an active galactic nucleus for ∼10⁷ yr, in which time it traverses ∼25 kpc.