Direct Abundance Maps and Radial Metallicity Gradients of two Galaxies at z~4-5 in the GARDEN Survey

We investigate galaxies in the GARDEN (Galaxies at All Redshifts Deciphered and Explained with the NIRSpec MSA) survey that show auroral emission lines, enabling spatially resolved measurements of electron temperature and direct oxygen abundances. Two galaxies have spectra suitable for this analysis: CANDELS 8005 at z=3.794 and CANDELS 7986 at z=4.702. For both, we measure auroral and key nebular emission-line fluxes across their full extent, allowing direct-method oxygen abundance determinations in individual spaxels. These observations demonstrate the viability of deep JWST/NIRSpec MSA spectroscopy for spatially resolved chemical analyses at high redshift, aided by weak nebular continua and low interstellar extinction. We derive global direct abundances of 12 + log(O/H) = 8.008 (+0.025, -0.027) for CANDELS 8005 and 7.89 (+0.027, -0.028) for CANDELS 7986. Emission-line diagnostics indicate neither galaxy hosts an active galactic nucleus. A first-order kinematic analysis suggests a potential merger in CANDELS 8005. The direct abundances agree with strong-line estimates from our data and recent high-redshift calibrations. We build emission line, radial velocity, strong-line abundance, electron temperature, and direct abundance maps for both galaxies. From these maps, we measure linear radial metallicity gradients of -0.111 (+0.026, -0.025) dex/kpc for CANDELS 8005 (statistically significant) and -0.093 +/- 0.088 dex/kpc for CANDELS 7986, where the large uncertainties limit significance. These results represent the first detection of a radial metallicity gradient from direct-method abundances with measurements taken in galaxies at z>0, supporting inside-out galaxy growth with feedback-regulated chemical enrichment.

💡 Research Summary

This paper presents the first spatially‑resolved, direct‑method oxygen abundance measurements in galaxies at redshifts greater than four, using deep JWST/NIRSpec micro‑shutter array (MSA) spectroscopy from the GARDEN (Galaxies at All Redshifts Deciphered and Explained with the NIRSpec MSA) survey. Out of 56 high‑redshift targets, two galaxies—CANDELS 8005 (z = 3.794) and CANDELS 7986 (z = 4.702)—showed sufficiently strong auroral lines (principally