Explaining the Weak Evolution of the High-redshift Mass–Metallicity Relation with Galaxy Burst Cycles

Recent observations suggest a nearly constant gas-phase mass–metallicity relation (MZR) at z ≳ 5, in agreement with many theoretical predictions. This lack of evolution contrasts with observations at z ≲ 3, which find an increasing normalization of the MZR with decreasing redshift. We analyze a high-redshift suite of FIRE-2 cosmological zoom-in simulations to identify the physical drivers of the MZR. Previous studies have explained the weak evolution of the high-redshift MZR in terms of weakly evolving or saturated gas fractions, but we find that this alone does not explain the evolution in FIRE-2. Instead, stellar feedback following intense bursts of star formation drives enriched gas out of galaxies, resetting their interstellar medium and separating their histories into distinct “burst cycles.” We develop the “reduced burst model,” a simplified gas-regulator model that successfully reproduces the simulated MZR and identifies the dominant drivers of its evolution. As redshift decreases, the metallicity of inflows within burst cycles increases at fixed stellar mass due to increased wind recycling of enriched gas. Meanwhile, the metal mass produced by stars per inflowing gas mass within these cycles decreases because of decreased star formation per gas mass inflowing into the galaxy. The effects of these two processes on the median metallicity largely cancel, holding the MZR constant for z = 5–12. At fixed stellar mass, the simulations predict lower gas metallicities at higher Hα-derived star formation rates, in qualitative agreement with the fundamental metallicity relation, but this effect is reduced in rest UV-selected samples.