Milky Way may have begun as thousands of galaxies, new simulations suggest

In the early days of the universe, our home looked very different from how it does today. Back then, the region in our universe that would eventually become our Milky Way neighborhood was a collection of thousands of smaller galaxies—some pumping out tons of new stars, others filled only with gas or littered with dead stars and black holes.

You can watch how these galaxies coalesced over time and formed the familiar spiral disk we see today, thanks to a new set of simulations by a group of scientists led by Harley Katz, assistant professor of astronomy and astrophysics at the University of Chicago.

The simulations, which took three years to run even on high-powered supercomputers, are the most detailed model to date of how a galaxy like the Milky Way might have evolved over the first several billion years of its existence.

“What does the Milky Way look like at what we call cosmic dawn?” said Katz. “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.”

The six papers from the project, which is named MEGATRON, provide new insight into the makeup of early galaxies and how the elements formed over time. The papers are published in The Open Journal of Astrophysics.

How the Milky Way evolves over time

Since we cannot travel to the dawn of the universe to study it, one of the only ways we have to understand how space evolved over time is to build detailed computer models based on what we know about the laws of nature.

“Essentially, we put in all of the physics we think is relevant—gravity, hydrodynamics, radiation, chemistry, etc.—and then let it evolve and see if it reproduces what we actually see when we look around us today,” said Katz.

New data coming in from the powerful James Webb Space Telescope has added a new dimension to our understanding of the universe. So three years ago, a team of scientists undertook a project to incorporate this newfound knowledge into a model of what our galaxy might have looked like back in the first billion years of its existence. Now they can compare it to the readings Webb gets today to see what matches and what’s missing.

If you run the clock back, the scientists said, you see a web of large and small galaxies of all different types, which will eventually merge into the modern Milky Way.

“We follow thousands of subsystems in the model and directly compute what they all would have looked like with our most powerful space telescopes, which is many orders of magnitude more than what had been simulated before,” said Katz. “Within those you see an incredible diversity. Some of them are bursting out in star formation, others are dead, others are in the process of dying.”

One unique finding was that the model predicts the existence of “galaxies” that don’t have any stars at all but still shine. Some of these might have once had stars that exploded or collapsed directly into black holes; others might have only ever contained gas.

Another discovery addresses a long-standing puzzle in stellar astrophysics. In our Milky Way, we typically see that smaller and fainter galaxies have less iron. However, this trend breaks down in extremely faint systems where the amount of iron appears constant, independent of mass. Thus far, simulations have been unable to reproduce this behavior.

The culprit, the simulation suggests, is explosions from an exotic type of star left over from right after the Big Bang. These stars, known as Population III stars, are so old that they are made of only the first elements that existed in the universe—hydrogen and helium—and when they explode, they can produce more iron than other supernovae do. If a galaxy is large enough, it will have enough gravity to hold on to this iron; but if it’s too small, the iron will be lost to space.

These Population III stars, in fact, are so old that none have ever been directly seen by telescopes. Another paper in the set is the first to show how these stars form in an environment like the Milky Way and predicts where these stars would be most likely to exist if any still do—which may help astronomers search for them.

The physics of stars and elements

A hallmark of these simulations is that they are much more complex than anything attempted before.

“What’s unique about our simulation is that it’s the first time we have modeled the enrichment of individual chemical elements from individual stars after the Big Bang, coupled to detailed models for gravity, chemistry, radiation and stellar processes,” said Katz.

This model, for example, was the first large-scale simulation to include a detailed computation of “non-equilibrium physics.” Most physics models assume, for the sake of simplicity, that a system’s chemistry is more or less in equilibrium. But of course, in the real universe, galaxies are constantly evolving, which often breaks this key assumption.

Scientists weren’t sure how much this would affect the end result—and including it in a simulation slows down the computation significantly.

However, two of the papers that are part of the study show that adding non-equilibrium physics made a real difference—especially when dealing with flows of gases around galaxies, known as the circumgalactic medium.

Adding this complexity allows scientists to more accurately interpret observations and to put limits on the number of possible answers to many questions about the formation of the universe.

“Looking at these results, it’s very clear that the physics happening right after the Big Bang has a direct impact on what we see today in the local universe,” said Katz.

“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.”

Who’s behind this story?

Gaby Clark

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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