GA-NIFS: AGN activity in a Ly α emitter within a triple-AGN system anchored by a passive galaxy at z = 3

Context. Massive quenched galaxies at z > 3 challenge models of early galaxy evolution, as their rapid formation and abrupt quenching require efficient feedback, which is often linked to active galactic nuclei (AGNs). The quiescent galaxy GS10578 at z = 3.064 is a key example of this population. Previous JWST/NIRSpec integral field unit (IFU) observations revealed an AGN-driven outflow and uncovered a compact pair of AGNs separated by ∼5 kpc. In addition, VLT/MUSE spectroscopy identified a third AGN candidate at a projected distance of ∼30 kpc, associated with a luminous Ly α emitter (LAE2) characterised by high-ionisation UV lines, although rest-frame optical diagnostics were not previously available. Aims. We aim to confirm the nature of LAE2 using rest-frame optical diagnostics enabled by new JWST observations, and to characterise the physical and ionisation properties of both LAE2 and a distinct nearby Ly α emitter (LAE1) that lacks any detectable continuum counterpart. Through this analysis, we investigated the interplay among low-mass satellites, black-hole growth, and the ionised environment of a quenched massive galaxy at high redshift. Methods. We analysed new JWST/NIRSpec IFU observations targeting the optical nebular lines of LAE1 and LAE2, including H β , [O III ], H α , and [N II ], complemented with VLT/MUSE IFU spectroscopy, as part of the GA-NIFS project. We extracted integrated and spatially resolved spectra, constructed emission-line maps, and used standard diagnostic diagrams to determine ionisation sources and kinematics. Results. LAE2 exhibits rest-frame UV–optical-line ratios fully consistent with an embedded AGN. Its [O III ] and H α emission displays a clumpy morphology and irregular, non-rotating kinematics on sub-kiloparsec scales. Except for Ly α , LAE1 remains undetected in all nebular lines and in JWST imaging; its Ly α emission is broad ( σ ∼ 200 km s −1 ) and asymmetric. The similarity of the LAE1 and LAE2 Ly α profiles in both velocity and flux suggests that LAE1 traces resonantly scattered emission powered by LAE2 rather than in situ star formation (although the latter cannot be completely ruled out). Conclusions. Our analysis reveals that the environment of GS10578 contains both multi-black-hole activity and gas structures on tens-of-kiloparsec scales, offering new insights into how feedback, accretion, and satellite interactions influence the late evolutionary stages of quenched massive galaxies.