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

Abstract 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.

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Journal
The Astrophysical Journal Letters
Published
2026-10-30
DOI
https://doi.org/10.3847/2041-8213/aea0cd
Primary Topic
Galaxies: Formation, Evolution, Phenomena
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article
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article

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

Archana Aravindan, Seiji Fujimoto, Jed McKinney, Ansh R. Gupta et al.
The Astrophysical Journal Letters
Galaxies: Formation, Evolution, Phenomena
article

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

Archana Aravindan, Seiji Fujimoto, Jed McKinney, Ansh R. Gupta, Hollis B. Akins, Stephanie M. Urbano Stawinski, Julián B. Muñoz, Michael Boylan-Kolchin, Kohei Inayoshi, Caitlin M. Casey, Olivia R. Cooper, Maximilien Franco, Erini Lambrides, Jeyhan S. Kartaltepe, John Chisholm, Andreas L. Faisst
article en

Abstract

Abstract 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.

The Astrophysical Journal LettersVol. 1011(1)
California Institute of Technology (US), Goddard Space Flight Center (US), University of California, Santa Barbara (US), Rochester Institute of Technology (US), University of Toronto (CA), Peking University (CN), Université Paris-Saclay (FR), Infrared Processing and Analysis Center (US), The University of Texas at Austin (US)
National Science Foundation, National Aeronautics and Space Administration, University of Texas at Austin, Space Telescope Science Institute, National Radio Astronomy Observatory, Korea Astronomy and Space Science Institute, National Astronomical Observatory of Japan, National Institutes of Natural Sciences
Openalex Percentile: Top 14%
Galaxies: Formation, Evolution, Phenomena
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