An Investigation into the Low-Mass Fundamental Metallicity Relation in the Local and High-z Universe
Recent JWST/NIRSpec observations have revealed high-$z$ star-forming galaxies depart from the Fundamental Metallicity Relation (FMR), yet the $z = 0$ FMR has not been well-characterized in the low-mass regime ($\rm log(M_{\star}/M_{\odot}) \lesssim 9$) for an appropriate comparison of low- and high-$z$ systems. We attempt to rectify this limitation through a meta-analysis, providing a local, observational comparison for future high-$z$ FMR studies. We analyzed common FMR fitting methods for $\sim 700$ [OIII]$λ4363$ emitters with $\rm log(M_{\star}/M_{\odot}) \lesssim 9$ at $z \sim 0$. We find no evidence of the FMR below $\rm log(M_{\star}/M_{\odot}) \lesssim 9$ through any method, suggesting that slowly-evolving, quasi-steady state gas reservoirs are not yet established. We simultaneously find a weak positive correlation between metallicity and star formation, and that these systems are gas-rich with substantial diversity in effective yields ($y_{\rm eff}$) spanning $\rm \sim 3~dex$. We demonstrate increasing $y_{\rm eff}$ correlates with decreasing FMR offsets, which in the context of the analytical and non-equilibrium gas models of Dalcanton et al. (2007), indicates a scenario where star formation bursts rapidly return and eject metals from the ISM before subsequent gas-balancing. Pristine infall diluting the ISM metal-content cannot lead to the $y_{\rm eff}$ diversity we measure, and thus is not the primary process behind FMR deviations. Our results suggest low-$\rm M_{\star}$ systems, regardless of redshift, depart from a steady-state gas reservoir shaping the canonical FMR, in which metallicity variations are primarily driven by star formation and enriched outflows. With this characterization, we demonstrate $z \gtrsim 3$ [OIII]$λ4363$ systems are indeed more metal-poor than $z \sim 0$ counterparts ($\rm Δ12+log(O/H) = 0.3~dex$) at fixed $\rm M_{\star}$.