The Physical Origin of Positive Metallicity Radial Gradients in High-redshift Galaxies: Insights from the FIRE-2 Cosmological Hydrodynamic Simulations

Creators: Sun, Xunda¹; Wang, Xin^{1, 2, 3}; Ma, Xiangcheng⁴; Wang, Kai⁵; Wetzel, Andrew⁶; Faucher-Giguère, Claude-André⁷; Hopkins, Philip F.⁸; Kereš, Dušan⁹; Graf, Russell L.⁶; Marszewski, Andrew⁷; Stern, Jonathan¹⁰; Sun, Guochao⁷; Sun, Lei¹; Thyme, Keyer¹¹

1. University of Chinese Academy of Sciences
2. National Astronomical Observatories
3. Beijing Normal University
4. University of California, Berkeley
5. Peking University
6. University of California, Davis
7. Northwestern University
8. California Institute of Technology
9. University of California, San Diego
10. Tel Aviv University
11. University of Chicago

Abstract

Using the FIRE-2 cosmological zoom-in simulations, we investigate the temporal evolution of gas-phase metallicity radial gradients of Milky Way–mass progenitors in the redshift range of 0.4 < z < 3. We pay special attention to the occurrence of positive (i.e., inverted) metallicity gradients—where metallicity increases with galactocentric radius. This trend, contrary to the more commonly observed negative radial gradients, has been frequently seen in recent spatially resolved grism observations. The rate of occurrence of positive gradients in FIRE-2 is about ∼7% for 0.4 < z < 3 and ∼13% at higher redshifts (1.5 < z < 3), broadly consistent with observations. Moreover, we investigate the correlations among galaxy metallicity gradient, stellar mass, star formation rate (SFR), and degree of rotational support. Metallicity gradients show a strong correlation with both sSFR and the rotational-to-dispersion velocity ratio (v_c/σ), implying that starbursts and kinematic morphology of galaxies play significant roles in shaping these gradients. The FIRE-2 simulations indicate that galaxies with high sSFR (log(sSFR[yr−1])≳−9.2) and weak rotational support (v_c/σ ≲ 1) are more likely—by ∼15%—to develop positive metallicity gradients. This trend is attributed to galaxy-scale gas flows driven by stellar feedback, which effectively redistribute metals within the interstellar medium. Our results support the important role of stellar feedback in governing the chemo-structural evolution and disk formation of Milky Way–mass galaxies at the cosmic noon epoch.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

We thank the anonymous referee for a very constructive report that helped improve the clarity of this paper. This work is supported by the National Natural Science Foundation of China (grant 12373009), the CAS Project for Young Scientists in Basic Research grant No. YSBR-062, the Fundamental Research Funds for the Central Universities, and the China Manned Space Program with grant No. CMS-CSST-2025-A06. X.W. acknowledges the support by the Xiaomi Young Talents Program, and the work carried out, in part, at the Swinburne University of Technology, sponsored by the ACAMAR visiting fellowship. A.W. received support from NSF, via CAREER award AST-2045928 and grant AST-2107772, and HST grant GO-16273 from STScI. C.A.F.G. was supported by NSF through grants AST-2108230 and AST-2307327; by NASA through grant 21-ATP21-0036; and by STScI through grant JWST-AR-03252.001-A.

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