Galaxy Metallicity Gradients in the Epoch of Reionization from the FIRE-2 Simulations

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Abstract We employ the high-redshift suite of FIRE-2 cosmological hydrodynamic zoom-in simulations to investigate the evolution of gas-phase metallicity radial gradients in galaxies in the epoch of reionization (EOR). Our sample consists of 22 galaxies spanning the redshift range z ∼ 10–5. We find that galaxies at z ∼ 10 exhibit a median metallicity gradient of −0.15 dex kpc −1 with substantial scatter, which gradually flattens to −0.1 dex kpc −1 at z ∼ 6, accompanied by a reduction in scatter. In the EOR, metallicity gradients correlate positively with stellar mass: more massive galaxies display flatter gradients with smaller scatter, broadly consistent with recent JWST observations. At fixed stellar mass, galaxies with a higher star formation rate (SFR) exhibit steeper negative gradients, while the specific SFR shows a strong anticorrelation with gradient slope. Because EOR galaxies in FIRE-2 generally lack significant rotational support, we adopt the ratio of the peak-to-peak velocity shear to twice the velocity dispersion (Δ v /2 σ ) as a proxy for the strength of gas flows. We find a strong positive correlation between metallicity gradients and Δ v /2 σ : galaxies with a lower Δ v /2 σ (i.e., weaker gas flows) tend to exhibit steeper negative gradients. Furthermore, galaxies with a steeper gradient display higher central SFR surface densities, suggesting localized star formation with inefficient interstellar medium mixing that drives inside-out chemical enrichment in galaxy evolution in the early Universe.

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