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

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

First listed 2026-09-03 | Last updated 2026-09-01

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 H$α$, a significant Balmer break, and no H$β$ 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 H$α$ with a velocity offset of $1245{\,\rm km\,s^{-1}}$ from the systemic narrow line velocity and with FWHM$\,\sim2500\,{\rm km\,s^{-1}}$. 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]$_{157\,μ\rm m}$ and [N II]$_{\rm 205\,μm}$ 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 $\sim10$ Myr to $\sim100$ 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.

Short digest

McKinney et al. present JWST/NIRSpec PRISM and G395M spectroscopy of AzTEC-1, the exceptionally bright sub-mm galaxy at z=4.34, finding a dust-attenuated 100–400 Myr-old stellar population and narrow-line BPT ratios consistent with an AGN. The central result is a broad Hα component with FWHM of about 2500 km s⁻¹ that is blueshifted by 1245 km s⁻¹ relative to the narrow systemic emission, with the resolved G395M fit strongly favoring an offset broad-line region rather than systemic broad Hα. The authors argue that AzTEC-1's smooth, post-starburst-like morphology and dense, unstable gas disk make a merger-delivered SMBH companion and stalled binary evolution plausible, while the lack of extended signatures across NIRSpec, NIRCam, and resolved ALMA data disfavors an ionized outflow interpretation. If confirmed, the system would offer a rare view of an obscured, high-redshift SMBH binary in the evolutionary regime relevant to low-frequency gravitational-wave sources.

Key figures to inspect

• Figure 4. The resolved NIRSpec/G395M Hα+[N II] fit is the paper's core observational evidence: it shows that allowing the broad Hα component to have an independent velocity is statistically preferred over a systemic broad-line model.
• Figure 5. The two-dimensional G395M analysis tests whether the unusual broad and narrow profiles are spatially distinct or extended. Its unresolved, point-source-like line emission is central to the argument against a galaxy-scale ionized outflow.
• Figure 8. This BPT comparison establishes that AzTEC-1's narrow-line ratios lie in the general AGN domain, providing the excitation diagnostic needed to interpret the offset broad Hα feature as connected to accreting black-hole activity.
• Figure 10. This synthesis plot places AzTEC-1's broad Hα velocity offset and width against known outflows and binary-SMBH candidates. It makes clear both why an extreme outflow remains a comparison case and why the absence of spatially extended emission is decisive for the binary interpretation.
• Figure 12. The black-hole-to-stellar-mass comparison translates the candidate-binary scenario into host-galaxy evolution, showing how the inferred companion or total binary mass would compare with local scaling relations and JWST AGN populations.