JWST Reveals a Candidate Supermassive Black Hole Binary at z=4.3 in the Brightest Sub-millimeter Galaxy in COSMOS-Web
Abstract
We present JWST/NIRSpec PRISM and G395M grating spectroscopy for AzTEC-1, a massive sub-mm 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. 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, raising the possibility for a binary supermassive black hole system. In this scenario, we assume that the lower mass black hole hosts a broad line region orbiting a quiescent primary. Evidence for an extended outflow is not found in the 2D spectrum, NIRCam imaging, resolved 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 indicates that massive gas clouds external to the candidate binary SMBH's orbit might have played a role in stalling infall from ∼10 Myr to ∼100 Myr through dynamical torques, which has been theorized to occur in the nuclei of massive galaxies like AzTEC-1. If the supermassive black hole binary is confirmed, AzTEC-1 would be an excellent laboratory into the astrophysics driving low-frequency gravitational wave detections.
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BibTeX
@misc{mckinney2026jwst,
title = {{JWST Reveals a Candidate Supermassive Black Hole Binary at z=4.3 in the Brightest Sub-millimeter Galaxy in COSMOS-Web}},
author = {Jed Mckinney and Ansh Gupta and Julian B Munoz 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},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.01711},
eprint = {2609.01711},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05758423v1},
}
RIS
TY - UNPB
TI - JWST Reveals a Candidate Supermassive Black Hole Binary at z=4.3 in the Brightest Sub-millimeter Galaxy in COSMOS-Web
AU - Jed Mckinney
AU - Ansh Gupta
AU - Julian B Munoz
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 - 2026/09/01
PB - HAL
DO - 10.48550/arxiv.2609.01711
UR - https://hal.science/hal-05758423v1
AB - We present JWST/NIRSpec PRISM and G395M grating spectroscopy for AzTEC-1, a massive sub-mm 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. 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, raising the possibility for a binary supermassive black hole system. In this scenario, we assume that the lower mass black hole hosts a broad line region orbiting a quiescent primary. Evidence for an extended outflow is not found in the 2D spectrum, NIRCam imaging, resolved 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 indicates that massive gas clouds external to the candidate binary SMBH's orbit might have played a role in stalling infall from ∼10 Myr to ∼100 Myr through dynamical torques, which has been theorized to occur in the nuclei of massive galaxies like AzTEC-1. If the supermassive black hole binary is confirmed, AzTEC-1 would be an excellent laboratory into the astrophysics driving low-frequency gravitational wave detections.
ER -