JWST Reveals a Candidate Supermassive Black Hole Binary at z = 4.3 in the Brightest Submillimeter Galaxy in COSMOS-Web

We present JWST/NIRSpec PRISM and G395M grating spectroscopy for AzTEC-1, a massive submillimeter bright galaxy at z = 4.34 in the COSMOS extragalactic field. The PRISM spectrum reveals strong H α , a significant Balmer break, and no H β detection, indicating a 100–400 Myr old stellar population and high dust attenuation. Baldwin–Phillips–Terlevich (BPT) line ratios indicate the presence of an active galactic nucleus (AGN). Decomposing narrow- and broad-line components, we recover broad, blueshifted H α with a velocity offset of 1245 km s −1 from the systemic narrow-line velocity and with FWHM ≈ 2500 km s −1 . AzTEC-1’s smooth morphology and stellar age is suggestive of a past merger-induced starburst period that would have brought in a second supermassive black hole (SMBH), raising the possibility for a binary SMBH system. In this scenario, we assume that the lower-mass black hole (BH) hosts a broad-line region orbiting a quiescent primary. Evidence for an extended outflow is not found in the 2D spectrum, NIRCam imaging, resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of dust continuum, or CO, [C ii ] 157 μ m , and [N ii ] 2 05 μ m kinematics. AzTEC-1’s high central gas-mass surface density and dynamically unstable gas disk indicate that massive gas clouds external to the candidate binary SMBH’s orbit might have played a role in stalling infall from O ( 10 Myr) to O ( 100 Myr) through dynamical torques, which has been theorized to occur in the nuclei of massive galaxies like AzTEC-1. If the SMBH binary is confirmed, AzTEC-1 would be an excellent laboratory into the astrophysics driving low-frequency gravitational-wave detections.