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

Abstract

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 $z\approx 8$ 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 ($⟨[\mathrm{Fe}/\mathrm{H}]⟩\approx -2.5$) at low stellar masses ($M_{\star }\le 10^5{\mathrm{M}}_{\odot }$). 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 ($\approx 10^7{\mathrm{M}}_{\odot }$) to retain their ejecta. We also predict a tail of $\approx 20\%$ of iron-deficient ($⟨[\mathrm{Fe}/\mathrm{H}]⟩\le -3$) 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 $z=0$.

Main text 13 pages, part of the Megatron initial paper release. Submitted to the Open Journal of Astrophysics, comments welcome!