Chaotic galaxies are star factories from another time

Most galaxies we see today are quiet, well-structured, and orderly. But not all play by those rules. Some are wild, chaotic, and packed with stars that are forming at a furious pace.

These rare, chaotic galaxies give astronomers a glimpse of what the universe looked like billions of years ago – and perhaps a hint of what’s in store for our own Milky Way.

Astronomers from the University of Arizona have taken a detailed look at these unusual chaotic galaxies, also known as luminous and ultra-luminous infrared galaxies, or LIRGs and ULIRGs.

These are incredibly bright in the infrared spectrum and are often caught in the middle of a galactic collision.

This study, part of a larger project called the Great Observatories All-sky LIRG Survey (GOALS), includes data from space telescopes such as Spitzer, Hubble, Chandra, and GALEX – and now, for the first time, the James Webb Space Telescope (JWST) as well.

The results of the study were presented during a press briefing at the 246th meeting of the American Astronomical Society on June 11.

According to Sean Linden, a research associate at the University of Arizona Steward Observatory, the findings will be published in an upcoming edition of the Astrophysical Journal.

Cosmic collisions cause chaotic galaxies

LIRGs and ULIRGs are not common in the universe close to Earth. Only 202 have been found within 1.3 billion light-years of us. What makes them special is that they are actively merging.

Two galaxies smash together, forming bizarre structures – double galactic centers, long tails of stars and, most notably, star-forming clumps that are far larger than anything in our galaxy.

“These galaxies are very clumpy, very different from the beautiful spiral galaxies that we see now, such as the Milky Way,” said Linden. “And we know from cosmological simulations that these clumps were the building blocks of galaxies in the early universe.”

In our galaxy, the largest clumps of new stars weigh about 1,000 solar masses. In LIRGs and ULIRGs, a single clump can hold a million times the mass of our Sun. And there are hundreds of thousands of these clumps in each galaxy.

“You can imagine a million Suns forming in one small, compact region, and within one of those galaxies, there are hundreds of thousands of such clumps,” Linden explained.

Chaotic galaxies act as star factories

Galactic mergers are messy. But in that chaos, galaxies give birth to stars at a much higher rate than usual. When two galaxies collide, gas clouds compress, temperatures soar, and star formation kicks into overdrive.

“These clumpy structures build up over time until they become incredibly massive, and if we want to understand them and how they actually contribute to galaxies evolving throughout cosmic time, we need to study them in detail,” Linden said.

This is exactly what the new JWST data makes possible. Earlier telescopes, like Hubble, had already spotted some of these clumps. But thick clouds of dust made it impossible to see what was going on inside them.

JWST’s infrared vision has now peeled back those layers, giving researchers their best look yet at the internal workings of these star factories.

When chaotic galaxies crashed and clumped

The universe used to be a more violent place. Billions of years ago, galaxies were packed closer together and collided more frequently. That meant more clumps, more chaos, and more extreme star formation.

“The universe used to be much more violent and extreme in the past, and it’s now settling down,” Linden said. “That’s why these rare examples of extreme galaxies no longer exist in the local universe, because, by and large, most galaxies have settled down as well.”

Even so, these rare chaotic galaxies offer astronomers a chance to study a phenomenon that has mostly disappeared from the modern universe. They serve as a kind of time machine – letting us peek into how galaxies like ours might have looked long ago.

“In a sense, you look at the local universe, and it gives you information about what would have happened 10 billion years ago,” said Linden.

When the Milky Way meets Andromeda

These observations might not just be a look into the past. They could also foreshadow the future.

The Milky Way is on a slow-motion collision course with the Andromeda galaxy. That merger will happen several billion years from now but, when it does, our quiet and orderly galaxy could roar back to life.

“As Andromeda gets closer and the pressure in the interstellar medium goes up, all of a sudden, the clumps that you will find that the Milky Way is forming will be more and more massive,” Linden noted.

The survey also confirmed long-held predictions from computer simulations. Disk-shaped galaxies like the Milky Way form stars in small, scattered clumps. But when galaxies merge, they create larger clumps and more intense star-forming regions.

“We’re now finding these massive clumps in the local universe,” Linden said. “We are beginning to complete the picture by comparing, for the first time, observations of massive clumps from both the nearby and the distant universe.”

These rare chaotic galaxies, once hidden behind thick veils of dust, are now offering scientists a clearer view of how cosmic structures behave under extreme conditions.

Furthermore, with the help of JWST, researchers are uncovering how collisions and chaos shaped the universe we see today – and how they might shape what comes next.

Information from a University of Arizona press release.

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