JWST Reveals a Candidate Supermassive Black Hole Binary at z = 4.3 in the Brightest Submillimeter Galaxy in COSMOS-Web
9 affiliations
- 1The University of Texas at Austin
- 2University of California Santa Barbara
- 3University of Colorado
- 4NASA-Goddard Space Flight Center
- 5Université Paris-Saclay
- 6University of Toronto
- 7Peking University
- 8Caltech/IPAC
- 9Rochester Institute of Technology
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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BibTeX
@article{mckinney2026jwst,
title = {{JWST Reveals a Candidate Supermassive Black Hole Binary at z = 4.3 in the Brightest Submillimeter Galaxy in COSMOS-Web}},
author = {Jed McKinney and Ansh Gupta and Julian B. Muñoz and John Chisholm and Caitlin M. Casey and Stephanie M. Urbano Stawinski and Olivia Cooper and Erini Lambrides and Hollis Akins and Maximilien Franco and Archana Aravindan and Seiji Fujimoto and Kohei Inayoshi and Andreas L. Faisst and Jeyhan S. Kartaltepe and Michael Boylan-Kolchin},
journal = {The Astrophysical Journal Letters},
year = {2026},
month = oct,
doi = {10.3847/2041-8213/aea0cd},
url = {https://iopscience.iop.org/article/10.3847/2041-8213/aea0cd},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - JWST Reveals a Candidate Supermassive Black Hole Binary at z = 4.3 in the Brightest Submillimeter Galaxy in COSMOS-Web
AU - Jed McKinney
AU - Ansh Gupta
AU - Julian B. Muñoz
AU - John Chisholm
AU - Caitlin M. Casey
AU - Stephanie M. Urbano Stawinski
AU - Olivia Cooper
AU - Erini Lambrides
AU - Hollis Akins
AU - Maximilien Franco
AU - Archana Aravindan
AU - Seiji Fujimoto
AU - Kohei Inayoshi
AU - Andreas L. Faisst
AU - Jeyhan S. Kartaltepe
AU - Michael Boylan-Kolchin
PY - 2026
DA - 2026/10/30
JO - The Astrophysical Journal Letters
DO - 10.3847/2041-8213/aea0cd
UR - https://iopscience.iop.org/article/10.3847/2041-8213/aea0cd
AB - 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.
SN - 2041-8213
SN - 2041-8205
ER -