New supercomputer simulations running across a three-year period predict that the neighborhood which became the Milky Way originated as thousands of smaller galaxies, including starless systems that shone only with gas or black holes.
MEGATRON Model Traces Cosmic Dawn
Led by Harley Katz, an assistant professor of astronomy and astrophysics at the University of Chicago, the MEGATRON project modeled the evolution of a galaxy resembling the Milky Way from roughly 180 million years after the Big Bang through its first two billion years. The simulation incorporated physics including gravity, hydrodynamics, radiation, and chemistry to compare outputs against real-world observations from instruments like the James Webb Space Telescope and the Hubble Space Telescope.
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.
Running the model in reverse revealed a complex web of large and small systems merging into the current spiral disk. The simulation tracked thousands of subsystems, showing an immense diversity ranging from active star-forming regions to dying or entirely dead systems. Hot purple gas cooled and collapsed to form stars, while ultraviolet light and glowing oxygen reheated the material in a continuous cycle.
The Discovery of Starless Galaxies and Iron Anomalies
The simulations identified a distinct class of systems containing no stars that still emitted light. While some may have hosted early stars that collapsed into black holes or exploded, others likely held only gas. These formations contributed raw material directly to the assembling Milky Way.
A second major finding addresses iron distribution in faint dwarf galaxies. Earlier models failed to replicate why iron amounts in the faintest systems appear flat and independent of mass. The MEGATRON runs indicate that Population III stars—the primordial generation composed strictly of hydrogen and helium—produced massive iron outputs during supernovae. Massive galaxies retained this iron via gravity, while smaller systems lost it to open space.
Non-Equilibrium Physics Alters Circumgalactic Medium Results
Unlike previous models that assumed chemical equilibrium, two papers in the project removed this shortcut to reflect real-world galactic behavior. Incorporating non-equilibrium physics increased computational overhead while altering results significantly in the circumgalactic medium, where gas flows around galaxies.
By a snapshot representing 12 billion years ago, the model successfully generated a rotating disk complete with heavy elements, dust, and stellar bands left behind by a major collision. Katz noted that while the physics of the early universe directly shaped the modern local universe, the remaining discrepancies between simulations and observations will guide future astronomical inquiries.
