PI: Rey

In October 2025, we delivered a major milestone with the release of the MEGATRON simulation suite and an accompanying series of five coordinated papers (Katz et al. 2025; Rey et al. 2025; Cadiou et al. 2025; Choustikov et al. 2025; Storck et al. 2025). Building on the earlier dp265 project and with contributions on dp373, this body of work represents one of the most ambitious and comprehensive efforts to model galaxy formation, and crucially, their spectroscopic observables at high redshift from first principles, with minimal reliance on empirical assumptions.

A central ambition of the dp373 programme is to move beyond studying cosmic dwan in isolation and instead connect it directly to the galaxies we observe today. The paper Rey et al. 2025b “how the first stars create an iron metallicity plateau in the smallest dwarf galaxies” provides a critical step toward this goal, offering one of the clearest demonstrations to date that the imprint of the first stars can survive across cosmic time.

This work extends the original MEGATRON simulation suite by incorporating a broader range of stellar evolution pathways and chemical enrichment channels, enabling robust predictions for the elemental abundance patterns preserved in stars observed in and around the Milky Way today. These new calculations, performed on DiRAC-Cosma as part of dp373, significantly expand the scientific reach of the project by linking early-Universe star formation to present-day galactic archaeology.

A striking and highly non-trivial result emerges. The simulations reproduce the observed stellar mass–metallicity relation of dwarf galaxies at z = 0, including a long-standing and poorly understood feature: a metallicity plateau at the lowest stellar masses. In particular, MEGATRON predicts an over-abundance of dwarf galaxies with iron abundances clustered around ⟨[Fe/H]⟩ ≈ −2.5, in excellent agreement with observations of Local Group systems.

Crucially, the origin of this feature lies not in late-time galaxy evolution, but in the physics of the first generation of stars. The simulations show that the plateau arises naturally from pair-instability supernovae produced by Population III stars, which inject large quantities of metals into their host haloes at the earliest epochs. Because MEGATRON self-consistently captures the build-up of the Lyman–Werner radiation background, these enrichment events occur preferentially in haloes massive enough to retain their metals, effectively establishing a universal metallicity floor for the smallest galaxies.

This result provides a rare and explicit causal link between high-redshift physics and low-redshift observations, directly addressing a central objective of the dp373 programme. It demonstrates that present-day dwarf galaxies are not merely relics of hierarchical assembly, but instead act as fossil records of cosmic dawn, preserving the chemical signatures of the first stars over billions of years of evolution.

Fig.1: Small galaxies forming in one of the Megatron simulations, with the chemical enrichment of the Universe shown in orange.