2025MNRAS.536...51D

We present NIRSpec/micro-shutter assembly observations from the JWST large-area survey WIDE, targeting the rest-frame UV-optical spectrum of Ulema, a radio-active galactic nucleus host at redshift $z=4.6348$. The low-resolution prism spectrum displays high equivalent width nebular emission, with remarkably high ratios of low-ionization species of oxygen, nitrogen, and sulphur, relative to hydrogen; auroral O$^+$ emission is clearly detected, possibly also C$^+$. From the high-resolution grating spectrum, we measure a gas velocity dispersion of $\sigma \sim 400$$\mathrm{km\, s^{-1}}$, broad enough to rule out star-forming gas in equilibrium in the gravitational potential of the galaxy. Diagnostics based on emission-line ratios suggest that the nebular emission is due to a shock which ran out of pre-shock gas. To infer the physical properties of the system, we model simultaneously the galaxy spectral energy distribution and shock-driven line emission under a Bayesian framework. We find a relatively low-mass, star-forming system ($M_\star = 1.4 \times 10^{10}~{\rm M}_\odot$, $\mathrm{SFR} = 70~{\rm M}_\odot ~{\rm yr^{-1}}$), where shock-driven emission contributes 50 per cent to the total H β luminosity. The nebular metallicity is near solar - three times higher than that predicted by the mass-metallicity relation at $z=4.6$, possibly related to fast-paced chemical evolution near the galaxy nucleus. We find no evidence for a recent decline in the star-formation rate of the galaxy, meaning that, already at this early epoch, fast radio-mode active galactic nucleus feedback was poorly coupled with the bulk of the star-forming gas; therefore, most of the feedback energy must end up in the galaxy halo, setting the stage for future quenching.