JWST/NIRSpec WIDE survey: a z = 4.6 low-mass star-forming galaxy hosting a jet-driven shock with low ionization and solar metallicity

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 $\mathrm{O}^{+}$ emission is clearly detected, possibly also $\mathrm{C}^{+}$ . From the high-resolution grating spectrum, we measure a gas velocity dispersion of $\sigma \sim 400\ \mathrm{km/s}$ , 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 ($M_{\ast} = 1.4 \times 10^{10}\ M_{\odot}$ , $\mathrm{SFR} = 70\ M_{\odot}/\mathrm{yr}$ ), where shock-driven emission contributes 50 per cent to the total $\mathrm{H}\beta$ 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.

Monthly Notices of the Royal Astronomical Society, Volume 536, Issue 1, pages 51-71