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

Martin P. Rey, Harley Katz, Corentin Cadiou, Mahsa Sanati, Oscar Agertz, Jeremy Blaizot, Alex J. Cameron, Nicholas Choustikov, Julien Devriendt, Uliana Hauk, Alexander P. Ji, Gareth C. Jones, Taysun Kimm, Isaac Laseter, Sergio Martin-Alvarez, Kosei Matsumoto, Autumn Pearce, Yves Revaz, Francisco Rodriguez Montero, Joki Rosdahl, Aayush Saxena, Adrianne Slyz, Richard Stiskalek, Anatole Storck, Oscar Veenema, Wonjae Yee

TL;DR

MEGATRON demonstrates that the iron metallicity plateau observed in ultra-faint dwarf galaxies arises from internal enrichment by high-mass Pop III PISNe in halos of order $M_{200} \sim 10^7\,M_\odot$, aided by a strong Lyman-Werner background that enables metal retention. By conducting four high-resolution cosmological radiation-hydrodynamics zoom-ins with identical initial conditions but varying Pop II feedback and IMF assumptions, the authors reproduce the slope and normalization of the local dwarf mass–metallicity relation, including a plateau at $M_\star \leq 10^5\,M_\odot$ and a tail of iron-deficient systems, with the plateau value around $\langle [\rm Fe / H] angle \approx -2.5$. The plateau emerges from the coupling of Pop III yields and the halo-scale metal retention in the presence of the LW background, with most dwarfs forming a single Pop III explosion that seeds Pop II star formation. These results establish a robust bridge between high-redshift chemical enrichment and present-day Galactic archaeology, offering a potential avenue to constrain the Pop III IMF and PISN yields via the iron-metallicity distributions of local UFDs, and motivate targeted JWST and future spectroscopic surveys.

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 $z\approx8$ 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 $z=0$.

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

TL;DR

MEGATRON demonstrates that the iron metallicity plateau observed in ultra-faint dwarf galaxies arises from internal enrichment by high-mass Pop III PISNe in halos of order , aided by a strong Lyman-Werner background that enables metal retention. By conducting four high-resolution cosmological radiation-hydrodynamics zoom-ins with identical initial conditions but varying Pop II feedback and IMF assumptions, the authors reproduce the slope and normalization of the local dwarf mass–metallicity relation, including a plateau at and a tail of iron-deficient systems, with the plateau value around . The plateau emerges from the coupling of Pop III yields and the halo-scale metal retention in the presence of the LW background, with most dwarfs forming a single Pop III explosion that seeds Pop II star formation. These results establish a robust bridge between high-redshift chemical enrichment and present-day Galactic archaeology, offering a potential avenue to constrain the Pop III IMF and PISN yields via the iron-metallicity distributions of local UFDs, and motivate targeted JWST and future spectroscopic surveys.

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 in a region that will collapse into a Milky-Way-like galaxy at , 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 , including an over-abundance of dwarfs along a flat plateau in metallicity () at low stellar masses (). 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 () to retain their ejecta. We also predict a tail of of iron-deficient () 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 .

Paper Structure

This paper contains 19 sections, 12 figures.

Table of Contents

  1. Introduction
  2. The Megatron high-redshift suite
  3. Proto-Milky Way initial conditions
  4. The high-redshift galaxy formation model
  5. The suite of four simulations
  6. Halo finding and dynamical future of high-redshift dwarfs
  7. The stellar mass-metallicity relation of dwarf galaxies
  8. The origin of the iron plateau
  9. The importance of Pop. III microphysics
  10. Chemical abundances beyond the plateau
  11. The coupling between the plateau and Pop. III star formation sites
  12. Should we expect agreement between local data and high-redshift simulations?
  13. Linking high- and low-redshift star formation histories and stellar masses
  14. Future environmental post-processing by a Milky Way's tidal field
  15. Discussion and conclusion
  16. ...and 4 more sections

Figures (12)

  • Figure 1: megatron cosmological radiation-hydrodynamics simulations (left panels, gas density, gas temperature, gas iron metallicity, LW background at $z\approx8$) model galaxy formation of a proto-galaxy that will turn into a Milky-Way-like galaxy at $z=0$ (right panels). High-redshift dwarf galaxies in megatron are mapped onto a dark matter only simulation (right, top panel, orange circles highlighting dwarf galaxies with $M_{\star} \leq 10^5 \, \textup{M}_\mathrm{\odot}$) to predict their dynamical future around the Milky-Way-like galaxy at $z=0$ (e.g. first infall, disrupted in the stellar halo, bound in orbiting satellites, bottom right panel, see Section \ref{['sec:lowzconnection']} for details of the particle tagging technique).
  • Figure 2: The stellar mass-mass metallicity relation of dwarf galaxies, observed at $z=0$ (grey points, Pace2025LVDatabase), observed in $z=3$ star-forming galaxies (Stanton2024b), and simulated megatron galaxies at $z\approx8$ (color points). The left panel shows all individual simulated dwarfs, while the right panel shows the mode in bins of $M_{\star}$. The agreement in slope, normalization, and scatter over $10^5 \leq M_{\star} \leq 10^8 \, \textup{M}_\mathrm{\odot}$ is striking. Only one simulation ('Efficient SF', purple) overshoots the data, due to inefficient galactic outflows in higher-$M_{\star}$ galaxies. All megatron simulations plateau in iron metallicities at low masses ($M_{\star} \leq 10^5\, \textup{M}_\mathrm{\odot})$, with an extended tail of iron-poor galaxies (Figure \ref{['fig:ufdplateau']}).
  • Figure 3: Same as Figure \ref{['fig:mstar-feh-suite']} but zooming on the low-mass end of the stellar mass-metallicity relation. In all simulations, the distribution of faint dwarf galaxies is strongly peaked around $\langle [\rm Fe / H] \rangle \approx -2.5$ over a broad range of $M_{\star}$, reproducing the observed 'plateau' of local dwarf galaxies (grey histogram and data points), and a long tail of iron-deficient galaxies. Both features emerge from our choices of Pop. III star formation and stellar evolution (Figure \ref{['fig:popiiinumbers']} and Figure \ref{['fig:popiiitype']}).
  • Figure 4: The number of Pop. III explosions within megatron dwarf galaxies broken down by their position on, above, or below the plateau (red, brown, and gold, respectively). Almost all faint dwarf galaxies undergo exactly one Pop. III explosion. A small fraction (2% of the total population) of dwarf galaxies are externally enriched by diluted ejecta from nearby objects and have had no Pop. III explosions. Those galaxies are exclusively found below the plateau but cannot explain the whole tail of iron-deficient dwarf galaxies.
  • Figure 5: Same as Figure \ref{['fig:popiiinumbers']}, but breaking down each sub-population of dwarfs by the exact progenitor of Pop. III explosions. megatron galaxies experience almost no Pop. III CCSNe and HNe explosions due to the choice of a top-heavy Pop. III IMF (first and second panels). But dwarf galaxies on the metallicity plateau are mostly enriched by high-mass PISNe (fourth panel), while galaxies below the plateau have a significant contribution from low-mass PISNe (third panel). This split is naturally explained by the vastly different iron yields in explosions of low-mass and high-mass PISNe (see text for discussions).
  • ...and 7 more figures