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

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 10 6 M ⊙ ≲ M ⋆ ≲ 10 10 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 A 100 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 ( A 100 = 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 ⋆ ≃ 10 10 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.