The mass-metallicity relation as a ruler for galaxy evolution: Insights from the James Webb Space Telescope

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
² Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, Largo Bruno Pontecorvo 3, Pisa I-56127, Italy
³ Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA
⁴ INAF-Osservatorio di Astrofisica e Scienza dello Spazio, via Gobetti 93/3, I-40129 Bologna, Italy

^⋆ Corresponding author: andrea.pallottini@unipi.it

Received: 31 July 2024
Accepted: 9 May 2025

Abstract

Context. Galaxy evolution emerges from the balance between cosmic gas accretion, fueling star formation, and supernova feedback, regulating metal enrichment of the interstellar medium. Hence, the relation between stellar mass (M_⋆) and gas metallicity (Z_g) is fundamental to understanding the physics of galaxies. High-quality spectroscopic JWST data enable accurate measurements of both M_⋆ and Z_g up to redshift z ≃ 10.

Aims. Our aims are to understand (i) the nature of the observed mass-metallicity relation (MZR), (ii) its connection with the star formation rate (SFR), (iii) the role played by SFR stochasticity (flickering), and (iv) how it is regulated by stellar feedback.

Methods. We compared the MZR obtained by the JADES, CEERS, and UNCOVER surveys, which comprise about 180 galaxies at z ≃ 3 − 10 with 10⁶ M_⊙ ≲ M_⋆ ≲ 10¹⁰ M_⊙, with ≃200 simulated galaxies in the same mass range from the SERRA high-resolution (≃20 pc) suite of cosmological radiation-hydrodynamic simulations. To interpret the MZR, we developed a minimal, physically motivated model of galaxy evolution that includes: cosmic accretion, possibly modulated with an amplitude A₁₀₀ on 100 Myr timescales; a time delay, t_d, between SFR and supernova feedback; and SN-driven outflows with a varying mass loading factor, ϵ_SN, which is normalized to the FIRE simulations predictions for ϵ_SN = 1.

Results. Using our minimal model, we find the observed “mean” MZR is reproduced for relatively inefficient outflows (ϵ_SN = 1/4), in line with findings from JADES. Matching the observed MZR “dispersion” across the full stellar mass range requires a delay time, t_d = 20 Myr, in addition to a significant modulation (A₁₀₀ = 1/3) of the accretion rate. Successful models are characterized by relatively low flickering (σ_SFR ≃ 0.2), corresponding to a metallicity dispersion of σ_Z ≃ 0.2. Such values are close but slightly lower than predicted from SERRA (σ_SFR ≃ 0.24, σ_Z ≃ 0.3), clarifying why SERRA shows a flatter trend with respect to the observations and some tension, especially at M_⋆ ≃ 10¹⁰ M_⊙.

Conclusions. The MZR appears to be very sensitive to SFR stochasticity. The minimal model predicts that high root mean square values (σ_SFR ≃ 0.5) result in a “chemical chaos” (i.e. σ_Z ≃ 1.4), virtually destroying the observed MZR. As a consequence, invoking a highly stochastic SFR (σ_SFR ≃ 0.8) to explain the overabundance of bright, super-early galaxies would lead to inconsistencies with the observed MZR.