JADES and SAPPHIRES: galaxy metamorphosis amidst a huge, luminous emission-line region

This is the authors' abstract. We don't add key points for this paper.

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 James Webb Space Telescope (JWST)/NIRCam imaging in [O iii]λ λ4959, 5007 and H α and spectroscopically confirmed with NIRCam/wide-field slitless spectroscopy, thanks to the pure-parallel programme Slitless Areal Pure Parallel HIgh-Redshift Emission Survey. The end-to-end velocity offset is Δ v=830± 130~km\, s⁻¹. Nebulae with such large sizes and high luminosities (25 pkpc diameter, L_ [O \, III]=1.2× 10¹⁰~L_) 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 (3σ). 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 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.