JADES and SAPPHIRES: Galaxy Metamorphosis Amidst a Huge, Luminous Emission-line Region

We report the discovery of a remarkably large and luminous line-emitting nebula extending on either side of the Balmer-break galaxy JADES-GS-518794 at z = 5.89, detected with JWST/NIRCam imaging in $[O\, {\small III}]\lambda \lambda$4959,5007 and H α and spectroscopically confirmed with NIRCam/WFSS, thanks to the pure-parallel programme SAPPHIRES. The end-to-end velocity offset is Δv = 830 ± 130 km s−1. Nebulae with such large sizes and high luminosities (25-pkpc diameter, $L_[O\, {\small III}]=1.2\times 10^{10}~\mathrm{L_\odot }$) are routinely observed around bright quasars, unlike JADES-GS-518794. With a stellar mass of 1010.1 M⊙, this galaxy is at the knee of the mass function at z = 6. Its star formation rate declined for some time (10-100 Myr prior to observation), followed by a recent (10 Myr) upturn. This system is part of a candidate large-scale galaxy overdensity, with an excess of Balmer-break galaxies compared to the field (3 σ). We discuss the possible origin of this nebula as material from a merger or gas expelled by an active galactic nucleus (AGN). The symmetry of the nebula, its bubble-like morphology, kinematics, high luminosity, and the extremely high equivalent width of $[O\, {\small III}]$ together favour the AGN interpretation. Intriguingly, there may be a physical connection between the presence of such a large, luminous nebula and the possible metamorphosis of the central galaxy towards quenching.