Summary
The MEGATRON simulations connect pair-instability supernovae from the first stars with a flat iron-abundance trend in the smallest dwarf galaxies. They also predict that about 20% of the simulated dwarfs are extremely iron-poor.
A set of high-resolution cosmological simulations called MEGATRON links explosions of the universe’s first stars to a flat iron-abundance trend in the smallest dwarf galaxies. The study reports that simulated galaxies with stellar masses at or below 100,000 times the Sun’s mass cluster around a mean iron abundance of [Fe/H] ≈ −2.5, and attributes the pattern to pair-instability supernovae from Population III stars.
A flat iron level at the lowest stellar masses
The researchers modelled galaxy formation in a region that is expected to develop into a Milky Way-like galaxy by the present day. The simulations follow the region’s evolution to about redshift 8 and track the formation of both Population III and Population II stars, along with feedback and chemical enrichment. Redshift 8 marks an early cosmic era; redshift 0 refers to the present day.
The study compares simulated dwarf galaxies’ stellar masses with their iron abundances. In the notation [Fe/H], iron relative to hydrogen is measured against the Sun on a logarithmic scale. The simulations reproduce the observed stellar mass–metallicity relation at redshift 0, the authors report. At the low-mass end, they find a plateau: the mean [Fe/H] stays near −2.5 rather than continuing to fall as stellar mass decreases.
The model also predicts a tail of especially iron-poor dwarfs. About 20% have a mean [Fe/H] at or below −3.
How the model connects the plateau to the first stars
Population III stars are the first generation of stars, formed before earlier stars had enriched their surroundings with heavier elements. MEGATRON’s explanation centres on pair-instability supernovae: powerful explosions associated with very massive stars that can disperse newly made elements, including iron.
In the simulations, radiation from the surrounding protogalaxy creates a strong Lyman–Werner background. This kind of ultraviolet radiation can break apart molecular hydrogen, an important coolant in early gas clouds. The authors find that the radiation helps ensure pair-instability supernovae occur in haloes massive enough—about 10 million times the Sun’s mass—to retain the explosions’ ejecta. The retained material can then contribute to the chemical enrichment of the dwarf galaxies that form there.
This proposed sequence connects conditions around the first stars to the iron content of much later, low-mass galaxies. It offers a way to interpret why some of the smallest dwarfs share a similar mean iron abundance, while a separate fraction remains far more iron-poor.
A prediction that persists in simulated satellites
The authors report that both the plateau and the iron-poor tail persist when they make large changes to assumptions about feedback from Population II stars, the later stellar generation. They also find the features among bound satellite galaxies surrounding the central galaxy at redshift 0.
These are results from numerical simulations, rather than direct observations of the first stars. The calculation follows a region destined to form a Milky Way-like galaxy, so its predictions describe that modelled environment. Within that setting, the iron pattern is linked to early pair-instability supernovae and remains present across the feedback variations tested in the study.