Rapid growth in a dual AGN during a gas-rich merger at z~4.5

Hyewon Suh, Roberto Decarli, Emanuele Paolo Farina, Julia Scharwächter, Mar Mezcua, Giorgio Lanzuisi, Federica Loiacono, Brian C. Lemaux, Sukyoung K. Yi, Stefano Marchesi, Marta Volonteri, Günther Hasinger, Francesca Civano, Anniek Gloudemans, Ena Choi, Yeonwoo Nam, Adi Foord, Silvia Onorato

First listed 2026-07-23 | Last updated 2026-07-21

Abstract

The late stages of galaxy mergers - when two supermassive black holes (SMBHs) reach kiloparsec-scale separations - represent a critical phase for understanding SMBH growth, galaxy co-evolution, and the progenitors of low-frequency gravitational waves. Yet confirmed close-separation (<3 kpc) dual active galactic nuclei (AGNs) remain confined to the local Universe, despite predictions that such systems should be common in the merger-rich early Universe. Here we report the discovery of LID-1166, a dual AGN with a projected separation of ~1.5 kpc at z~4.5, representing the first such system known beyond the local Universe. Spatially resolved JWST/NIRSpec integral-field spectroscopy reveals two spatially and kinematically distinct narrow-line emission components with a line-of-sight velocity offset of ~ -164 km/s. Both components additionally exhibit broad Ha emission, including a compact off-nuclei broad Ha component associated with the companion source, confirming two actively accreting SMBHs. Independent ALMA [CII] 158 um observations reveal corresponding spatially and kinematically distinct cold gas components associated with the same nuclei, demonstrating that the system is a gas-rich merger. While undergoing super-Eddington accretion during a late-stage merger, the SMBHs already lie on the local black hole-host mass relation for massive elliptical galaxies within the uncertainties, suggesting that rapid, obscured growth may help establish this relation early in cosmic history. LID-1166 reveals a previously hidden phase of SMBH growth and points to a missing population of heavily obscured, merger-driven dual AGNs at high redshift.

Short digest

Suh et al. report LID-1166, a heavily obscured, X-ray-selected dual AGN at z~4.5 whose two nuclei are separated by just ~1.5 kpc, making it the first confirmed sub-3-kpc dual AGN beyond the local Universe. JWST/NIRSpec IFU data resolve distinct narrow Hα components offset by ~-164 km/s and detect broad Hα from both nuclei, while ALMA [CII] independently finds matched spatial and kinematic cold-gas components, establishing a gas-rich late-stage merger rather than a lensed single source. Broad-line mass estimates of log(MBH/M⊙)~7.8 and 7.2, together with a system-integrated Eddington ratio of 3.7, identify rapid super-Eddington growth in a phase that may help place massive black holes on the local black-hole–host relation early and expose a missing population of obscured merger-driven dual AGN.

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