The Milky Way began as a collection of thousands of smaller galaxies, according to a new simulation project led by the University of Chicago.
MEGATRON Project Reconstructs Cosmic Dawn
Led by Harley Katz, an assistant professor of astronomy and astrophysics at the University of Chicago, the research team spent three years running simulations on high-powered supercomputers. The project, named MEGATRON, models the complex physical processes—including gravity, radiation, chemistry, and hydrodynamics—that transformed a chaotic web of smaller systems into the spiral disk observed today. By incorporating these factors, the researchers aimed to determine if their model could reproduce the conditions observed in the modern universe.
What does the Milky Way look like at what we call cosmic dawn? For the first time, we can directly predict what the early Milky Way would have looked like to telescopes like Hubble or the James Webb Space Telescope.
Harley Katz, assistant professor of astronomy and astrophysics at the University of Chicago
Diversity Among Early Galactic Subsystems
The simulation tracks thousands of subsystems that eventually merged to form the modern Milky Way. The researchers noted an incredible diversity
among these early structures. According to Katz, the team computed the visual appearance of these systems at a level of detail many orders of magnitude higher than previous simulations. While some were actively producing stars, others were filled only with gas or littered with dead stars and black holes. The model further indicates that even at a young age, the galaxy could have developed a rotating disk containing significant amounts of dust and heavy elements.
Solving the Iron Puzzle with Population III Stars
The MEGATRON project addresses a persistent challenge in astrophysics: the iron metallicity plateau in small dwarf galaxies. While iron levels generally scale with mass, this trend breaks down in extremely faint systems where iron levels remain constant. Previous simulations have been unable to replicate this behavior. The study by Martin Rey and colleagues, titled MEGATRON: how the first stars can create an iron metallicity plateau in the smallest dwarf galaxies, identifies the culprit as explosions of Population III stars.
These ancient stars, formed immediately after the Big Bang, consisted solely of hydrogen and helium. When they exploded, they generated higher iron yields than subsequent supernovae. The simulation suggests that while larger galaxies possess sufficient gravitational pull to retain these metals, smaller dwarf galaxies lose them to the surrounding space, resulting in the observed constant iron levels.
Advancements in Non-Equilibrium Physics
A technical hallmark of the MEGATRON project is its implementation of non-equilibrium physics. While most simulations rely on the assumption that a system’s chemistry remains in equilibrium, the researchers noted that the constant evolution of galaxies frequently invalidates this premise. By computing non-equilibrium effects alongside local radiation fields, the team provided a more accurate view of the circumgalactic medium at cosmic noon, as detailed in the paper by Corentin Cadiou et al.
Comprehensive Insights into Galactic Evolution
The six papers published on September 30 provide a multifaceted view of the early universe.