New Simulations Reveal Milky Way Formed from Thousands of Early Galaxies

Before taking its current form as a luminous spiral galaxy, the Milky Way existed as a chaotic assembly of thousands of distinct smaller galaxies, according to new research. The findings indicate that within the first 2 billion years following the Big Bang, the region of the universe destined to become our galaxy was densely populated with numerous small galaxies of varying sizes and shapes. Over eons, gravitational interactions caused these celestial bodies to collide and merge, eventually coalescing into the single galaxy we inhabit today.

“Looking at these results, it’s very clear that the physics happening right after the Big Bang has direct impact on what we see today in the local universe,” said Harley Katz, an assistant professor of astronomy and astrophysics at the University of Chicago and co-author of the study. Astronomers rely on sophisticated computer models encoding the laws of nature to trace cosmic evolution, and this new work represents the most detailed simulation yet of how a galaxy like the Milky Way came to be.

The research emerges as telescopes like the James Webb Space Telescope (JWST) uncover anomalies that challenge existing models. JWST has detected unexpectedly bright early galaxies and a new category of compact objects known as “Little Red Dots.” These discoveries suggest that current computer simulations require updates to account for the complex physics of the early universe.

To address this gap, researchers developed a suite of supercomputer simulations named Megatron. Over three years of processing, the team traced ancient gas, starlight, and chemical processes from 180 million years to 2 billion years post-Big Bang. The simulations predict distinct light signatures for virtual galaxies, allowing scientists to directly compare the data with JWST spectral observations. This cross-referencing helps identify gaps in previous modeling efforts.

The Megatron project also tracks the life cycles of the first stars, which formed from pristine gas, lived briefly, and died in violent explosions. These deaths seeded the surrounding environment with heavy elements such as carbon, oxygen, and iron, creating the building blocks necessary for future stars, planets, and life. By comparing these simulations with JWST observations and chemical traces found in ancient stars, researchers can better understand stellar enrichment processes.

“Together, these complementary observations allow us to test competing models of the first stars in ways that weren’t previously possible,” said Martin Rey, a theoretical astrophysicist at the University of Bath in the U.K. The scientists published their findings on Sept. 30 as part of six papers in The Open Journal of Astrophysics.

While the simulation ends 2 billion years after the Big Bang, the Milky Way continued to grow through mergers long after that period. The most recent massive collision, involving the Sagittarius dwarf galaxy, began more than 6 billion years ago and is still ongoing. Recent estimates also suggest the Milky Way may be larger, heavier, and more lopsided than previously understood.

Katz noted that while the models help align simulations with observations, they also highlight areas requiring further investigation. “But there are also things we’re not getting right, which is interesting too — what are the parts we’re still missing? That can lead you into new directions and new questions,” he said. The U.K. portion of the Megatron collaboration is now preparing next-generation simulations that will incorporate additional physics, such as the behavior of active black holes, which JWST has shown to be surprisingly common in the early universe.