JWST Observations of the Double Nucleus in NGC 4486B: Possible Evidence for a Recent Binary SMBH Merger and Recoil

A recent study of the compact elliptical galaxy NGC 4486B using JWST Near-Infrared Spectrograph integral-field-unit-mode kinematics confirmed a supermassive black hole (SMBH) of mass MBH=3.6±0.7×108 (∼8% of the stellar mass). In addition to its double nucleus, the nuclear kinematics show pronounced asymmetries: a velocity dispersion peak displaced by 6 pc from the galaxy center and a ∼16 km s−1 offset in the mean stellar line-of-sight velocity near the SMBH. We examine the origin of the 12 pc double nucleus and show that the observations favor an SMBH surrounded by an eccentric nuclear disk (END). One proposed formation pathway for ENDs involves a gravitational-wave (GW) recoil of the SMBH following a binary merger. Our orbit superposition models contain ∼50% retrograde stars at the edge of the nuclear region, in agreement with END formation simulations. We infer a premerger mass ratio q > 0.15 and a recoil kick of ∼340 km s−1. Our N-body simulations show that with such a kick, the SMBH returns to the center within ∼30 Myr. Its flat central core is also consistent with earlier “binary black hole scouring.” We test two alternative mechanisms—buoyancy-driven oscillations and a premerger SMBH binary—but neither reproduces the observed offsets, favoring the GW kick scenario. Our N-body simulations also show that a prograde SMBH binary in a rotating host can become trapped in a corotation resonance, delaying coalescence. Although NGC 4486B is an old galaxy near the Virgo Cluster center, its SMBH appears to have merged only recently, making its nucleus a rare nearby laboratory for postmerger SMBH dynamics.