Astronomers have surveyed massive, dense star factories, unlike any found in the Milky Way, in a large number of galaxies across the local universe. The findings provide a rare glimpse into processes shaping galaxies in the very early universe and possibly the Milky Way a few billion years from now. 

Known as luminous and ultra-luminous infrared galaxies, or LIRGs and ULIRGs, these galaxies are relatively rare in the local universe, with only 202 known within 400 megaparsecs (1.3 billion light-years) from Earth, according to Sean Linden, a research associate at the University of Arizona Steward Observatory, who presented the findings during a press briefing at the 246th meeting of the American Astronomical Society on June 11.

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LIRGs and ULIRGs differ from spiral galaxies like the Milky Way in that they are in the process of merging with other galaxies. Most exhibit features such as two galactic nuclei instead of one or extended "tails" as gravity stretches and deforms the two objects. And unlike "modern" galaxies, they contain "clumps" – dense regions brimming with newborn stars, much more massive than anything found in "typical," evolved galaxies that are not undergoing mergers. 

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

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Astronomers are interested in LIRGs and ULIRGs because they serve as windows into a distant past when the universe was much younger and galaxies were much less evolved and crashed into each other much more frequently than today. 

This is where the Great Observatories All-sky LIRG Survey comes in, or GOALS for short. It combines imaging and spectroscopic data from NASA's Spitzer, Hubble, Chandra and GALEX spaceborne observatories in a comprehensive study of more than 200 of the most luminous infrared-selected galaxies in the local universe. Now, infrared observations with NASA's James Webb Space Telescope have provided the most complete census of these galaxies. Running from October 2023 until September 2024, the survey is the only of its kind. The team plans to publish the results in a forthcoming issue of The Astrophysical Journal.

"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 said. 

For comparison, the most massive young clumps in the Milky Way have masses of about 1,000 suns and, on average, one star is born each year. 

When two galaxies collide and merge, star formation rates increase dramatically, Linden explained, resulting in the massive clumps that are not seen in other galaxies that are not undergoing mergers. 

"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.

Although star-forming clumps had already been observed with the Hubble Space Telescope, only the infrared capabilities of JWST allowed astronomers to pull aside the veils of thick dust that had prevented them from obtaining a more detailed look at these features. 

The survey results also confirm predictions of galaxy evolution based on simulations done by supercomputers, which predicted that "typical," disk-like galaxies contain fewer clumps of star formation, and most of the star formation happens in small clumps, as seen in the Milky Way today. Mergers produce bigger clumps, and more of them, and more of the star formation takes place in the massive clumps.

"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." 

Being able to discern previously hidden details in these unusually massive star-forming clumps helps researchers better understand how these features and their host galaxies evolved over time, essentially providing a natural laboratory for a type of galaxy that for the most part no longer exists in the universe except for its most distant, outer regions. 

"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, whose work focused on imaging the clumps and the star clusters, and who led the data acquisition, reduction and analysis. 

The early universe was much denser, he explained, and mergers between galaxies happened much more frequently, producing massive star-forming clumps. As the universe evolved and space expanded, the galaxies became more and more like the Milky Way and the mature spiral galaxies we see today. 

"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."