Simulations suggest the faintest galaxies near the Milky Way share one iron level because a single giant first star exploded in each.

Smaller galaxies usually hold less iron, so the faintest ones circling the Milky Way should be the poorest of all. They aren’t. Below a certain size, they all carry about the same small share of iron, and computer models have struggled for years to explain why.

New simulations suggest an answer. In most of these tiny galaxies, the iron came from a single explosion of one giant star from the Universe’s first generation.

The simulations come from MEGATRON, a project led by Martin Rey of the University of Bath with collaborators at the University of Chicago and the Institut d’Astrophysique de Paris (IAP).

They’re computer models rather than telescope observations, and the result depends on assumptions that remain uncertain about the first stars.

Tiny galaxies share one iron level

Every carbon, oxygen and iron atom around us was made inside a star, Rey said. The very first stars formed from hydrogen and helium alone, and their explosions started that process.

In an interview with Earth.com, Rey explained why the oldest surviving stars matter. “Those old stars are basically a fossil record, preserving the chemical fingerprints of events that happened over 13 billion years ago,” he said.

Many of those stars belong to ultra-faint dwarf galaxies, the dimmest galaxies orbiting the Milky Way. Each holds no more than about 100,000 times the Sun’s mass in stars. They stopped making new stars long ago, so astronomers treat them as relics of the early universe.

Among larger dwarf galaxies, iron drops as size drops. For the smallest ones, however, that trend flattens out. Their stars average roughly one three-hundredth of the Sun’s share of iron, and astronomers call this the iron plateau.

Earlier simulations had trouble giving such small galaxies that much iron.

Building a young Milky Way

The MEGATRON team simulated a patch of the early Universe that will grow into a galaxy with the Milky Way’s mass. The program follows gas, starlight and chemistry at once, keeping count of more than 80 kinds of atoms and molecules.

That matters because the project aims to connect two views of the early Universe. The James Webb Space Telescope sees young galaxies directly, while old stars nearby keep a chemical record of the same era.

Four versions started from the same point. Each used different assumptions about the second generation of stars, which formed after the first stars had added heavier elements to the gas. All four used the same recipe for the first stars.

Together, the runs produced more than 500 small galaxies. They end about 12 billion years before the present.

One first star explosion per galaxy

In all four versions, most of the smallest galaxies ended up with the iron level astronomers measure in real ones. About 78 percent fell inside the plateau’s range, between a thousandth and a hundredth of the Sun’s share.

Since only the second-generation recipe changed between runs, the authors took that as a sign that the first stars set the pattern.

Most of the small galaxies had exactly one first-generation star explode inside them. Those on the plateau mostly got their iron from stars, between 160 and 300 times the Sun’s mass, which blow themselves apart.

Each of those explosions makes about five times the Sun’s mass in iron, according to the published estimates the team used.

Asked what surprised him most, Rey told Earth.com, “In our simulations, this plateau falls out naturally from the physics of the first stars.” The team hadn’t gone looking for it.

“The simulations were never designed to reproduce it and we had not even planned to explore or look into this question,” he said. One of his collaborators suggested making the figure out of curiosity.

Heavier clumps held the iron

An explosion that powerful can blast its debris out of a small galaxy. In MEGATRON, that mostly didn’t happen.

Ultraviolet light from the young central galaxy kept gas from forming the hydrogen molecules it needs to cool down and collapse into stars. So the first stars couldn’t form in the smallest clumps of matter.

They formed instead in heavier clumps of dark matter and gas, about 10 million times the Sun’s mass, with enough gravity to hold on to the debris. Later stars then formed from that iron-enriched gas.

The authors checked this with a simple calculation. Spreading five Suns’ worth of iron through the gas in one of those clumps matches the plateau’s average level.

A lighter giant star, around 140 times the Sun’s mass, makes about a tenth as much iron, which leaves its galaxy well below the plateau.

A poorer group of galaxies is missing

About 22 percent of the small simulated galaxies fall below the plateau, with less than a thousandth of the Sun’s share of iron. No ultra-faint galaxy measured so far is that poor.

If a fifth of them really were, the chance of finding none among the 38 measured would be about 0.02 percent, the authors calculated. That counts against the prediction.

Still, the census is thin. Many of these galaxies have iron measured in only a few stars. One of them, Pictor II, averages right at the plateau’s lower edge, and it holds a star with almost no iron.

Both groups would still exist today, according to the team’s forecast. Most small galaxies get torn apart and spread into the faint cloud of stars around a galaxy like the Milky Way, called the halo.

In a companion study led by Anatole Storck, the team found that 75 to 80 percent of the first stars’ remains end up there, Rey said.

“So the debris of the very first generation of stars is, quite literally, still out there around us,” Rey said.

What remains unknown

MEGATRON’s plateau rests on three assumptions, and the authors wrote that each is uncertain. Ultraviolet light from the young galaxy has to be strong. The first stars have to be mostly giants. And each giant explosion has to make a lot of iron.

That second assumption is debated, since some newer models suggest the first stars were lighter. The simulations also make much less nitrogen than astronomers measure in real old stars, and the simulated young galaxy may not match the Milky Way’s early history.

The Vera C. Rubin Observatory is expected to find many new faint galaxies, and the next generation of very large telescopes should measure the iron in their stars.

Rey has been awarded 40 million hours of processing time on the U.K.’s national supercomputers for the next round. He compared that to running five million laptops for a year.

With them, his team will test “which assumptions about the first stars match local data, and more importantly which do not,” he said.

One galaxy found below the plateau would fit the prediction. If surveys keep finding none, the first stars may have been even heavier than assumed, or the plateau has another cause.

The full study was published in the journal The Open Journal of Astrophysics.

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