JWST Reveals a Candidate Supermassive Black Hole Binary at z = 4.3 in the Brightest Submillimeter Galaxy in COSMOS-Web
November 2026 • 2026ApJ..1011L..35M
Abstract
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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 Hα, a significant Balmer break, and no Hβ 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 Hα 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 μm, and [N II]205 μm 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
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