MEGATRON: how the first stars can create an iron metallicity plateau in the smallest dwarf galaxies

We study the stellar mass-iron metallicity relation of dwarf galaxies in the new high-resolution MEGATRON cosmological radiation-hydrodynamics simulations. These simulations model galaxy formation up to \(z\approx8\) in a region that will collapse into a Milky-Way-like galaxy at \(z=0\), while self-consistently tracking Population III and II (Pop.~III, Pop.~II) star formation, feedback and chemical enrichment. MEGATRON dwarf galaxies are in excellent agreement with the observed stellar mass-metallicity relation at \(z=0\), including an over-abundance of dwarfs along a flat plateau in metallicity (\(\langle [\rm{Fe}/\rm{H}] \rangle \approx -2.5\)) at low stellar masses (\(M_{\star} \leq 10^5 \, \rm{M}_{\odot}\)). We tie this feature to the chemical enrichment of dwarf galaxies by Pop. III pair-instability supernova (PISN) explosions. The strong Lyman-Werner background (LW) from the protogalaxy ensures that PISNe occur in haloes massive enough (\(\approx 10^7\, \rm{M}_{\odot}\)) to retain their ejecta. We also predict a tail of \(\approx 20\%\) of iron-deficient (\(\langle [\rm{Fe}/\rm{H}] \rangle \leq - 3\)) dwarf galaxies. We show that both plateau and tail (i) are robust to large variations in Pop.~II feedback assumptions, and (ii) survive in bound satellites surrounding the central galaxy at \(z=0\).