Astronomers have identified massive, dense star-forming regions, unseen in the Milky Way, within numerous galaxies in our local universe. These observations provide a unique window into the processes that shaped galaxies in the early universe and potentially offer a glimpse into the Milky Way’s future billions of years from now.

These galaxies,known as luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs),are relatively scarce in our cosmic neighborhood. According to sean Linden, a research associate at the University of Arizona Steward Observatory, onyl 202 such galaxies have been identified within 400 megaparsecs (1.3 billion light-years) of Earth. Linden presented these findings at the 246th meeting of the American Astronomical Society on June 11.

LIRGs and ULIRGs differ significantly from spiral galaxies like the Milky Way as they are undergoing mergers with other galaxies.These merging galaxies often exhibit distinctive features, such as two galactic nuclei or extended “tails” caused by gravitational forces.Unlike “modern” galaxies, they also contain “clumps,” which are dense regions packed with newborn stars far more massive than those found in evolved galaxies not experiencing mergers.

“These galaxies are very clumpy, very different from the lovely 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.”

Astronomers are particularly interested in LIRGs and ULIRGs as they offer a glimpse into the distant past when the universe was younger, galaxies were less developed, and collisions were more frequent.

The Great Observatories All-sky LIRG Survey (GOALS) is a project that combines data from NASA’s Spitzer, Hubble, Chandra, and GALEX spaceborne observatories to study over 200 of the most luminous infrared-selected galaxies in the local universe. Recent infrared observations from NASA’s James Webb Space Telescope (JWST) have provided the most comprehensive view of these galaxies to date. This survey, which runs from October 2023 to September 2024, is the only one of its kind. The research team plans to publish their findings 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.

In comparison, the most massive young clumps in the Milky Way contain about 1,000 suns, with an average of one star born each year.

Linden explained that when galaxies collide and merge, star formation rates increase dramatically, leading to the formation of massive clumps not seen in galaxies that are not merging.

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

While star-forming clumps had been previously observed with the Hubble Space telescope, JWST’s infrared capabilities have allowed astronomers to penetrate the thick dust clouds and obtain a more detailed view of these features.

The survey results also support computer simulations of galaxy evolution, which predicted that “typical” disk-like galaxies would have fewer star formation clumps, with most star formation occurring in small clumps, as observed in the Milky Way today. Mergers, conversely, produce larger and more numerous clumps, with a greater proportion of star formation taking place within these 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.”

by resolving previously hidden details within these unusually massive star-forming clumps, researchers can better understand the evolution of these features and their host galaxies. These galaxies serve as a natural laboratory for studying a type of galaxy that largely no longer exists in the universe, accept in its most distant 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, who focused on imaging the clumps and star clusters and led the data acquisition, reduction, and analysis.

He explained that the early universe was much denser, and galaxy mergers were more frequent, resulting in the formation of massive star-forming clumps.As the universe expanded, galaxies evolved to resemble the Milky Way and other mature spiral galaxies.

“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,as,by and large,most galaxies have settled down as well.”

Along with providing insights into the past, these galaxies also offer clues about the future. Linden noted that the Milky Way and Andromeda galaxies are expected to collide in several billion years, potentially triggering another period of intense star formation in both galaxies.

“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.”