Spatially resolved emission lines in galaxies at 4 ≤ z < 10 from the JADES survey: Evidence for enhanced central star formation

Abstract

We present the first statistical investigation of spatially resolved emission-line properties in a sample of 63 low-mass galaxies at 4 ≤ z < 10 using James Webb Space Telescope (JWST)/NIRSpec Micro Shutter Assembly (MSA) data from the JWST Advanced Deep Extragalactic (JADES) survey, focusing on deep, spatially resolved spectroscopy in the GOODS-S extragalactic field. By performing a stacking of the 2D spectra of the galaxies in our sample, we find an increasing or flat radial trend with increasing radius for [O III]λ5007/Hβ and a decreasing trend for the blended spectral complex (Hη + [Ne III]λ3869 + He Iλ3889 + Hζ)/[O II]λ3727 (3−4σ significance). These results are still valid when stacking the sample in two redshift bins (i.e. 4 ≤ z < 5.5 and 5.5 ≤ z < 10). The comparison with star-formation photoionisation models suggests that the ionisation parameter increases by ∼0.5 dex with redshift. Under the hypothesis that radial variations in (Hη + [Ne III]λ3869 + He Iλ3889 + Hζ)/[O II]λ3727 are dominated by trends in [Ne III]λ3869/[O II]λ3727, we find a tentative metallicity gradient that increases with radius (i.e. ‘inverted’) in both redshift bins. Moreover, our analysis reveals strong negative gradients for the equivalent width of Hβ (7σ significance). This trend persists even after removing known active galactic nucleus candidates, and is therefore consistent with a radial gradient primarily in stellar age and secondarily in metallicity. Taken together, our results suggest that the sample is dominated by active central star formation, with possibly inverted metallicity gradients sustained by recent episodes of accretion of pristine gas or strong radial flows. Deeper observations and larger samples are needed to confirm these preliminary results and to validate our interpretation.