Meet the Parents: The Progenitor Binary for the Supermassive Black Hole Candidate in E1821+643

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 × 1012 M ⊙ 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 m1∼1.9−0.4+0.5×109M⊙ , m2∼8.1−3.2+3.9×108M⊙ (90% credible intervals). Given our model, the black holes in this binary were likely to be spinning rapidly with dimensionless spin magnitudes of χ1=0.87−0.26+0.11 , χ2=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 × 109 M ⊙ 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.