The James Webb Space Telescope has yielded the largest galaxy group sample ever observed after scanning a region known as the COSMOS Web, offering a large-scale look at the universe and illuminating galaxy evolution over time.
The new Webb images offer astronomers a glimpse at a portion of the universe so distant and so large that it took between one and twelve billion years for their light to reach Earth. The massive data set reveals about 1,700 galaxy groups, with one of them more than 6 billion light-years away claiming the title of the European Space Agency‘s picture of the month.
“We’re able to actually observe some of the first galaxies formed in the universe,” said Ghassem Gozaliasl of Aalto University, and head of the galaxy groups detection team, who led the study. “We detected 1,678 galaxy groups or proto-clusters – the largest and deepest sample of galaxy groups ever detected – with the James Webb Space Telescope.”
“With this sample, we can study the evolution of galaxies in groups over the past 12 billion years of cosmic time,” Gozaliasl said.
Since its activation in 2022, the James Webb Space Telescope has increased the distance and clarity with which astronomers can peer into the cosmos, thanks to its high resolution and sensitivity. This also gives astronomers a deeper look at the universe’s past, as the vast distances involved can take billions of years even for light to cross, bringing images to Earth of what once was at the edges of the universe. Those faint, distant galaxies may be a billion times dimmer than the human eye can detect, yet such strict limitations do not hamper the advanced instrumentation aboard the space telescope.
Galaxy Clusters
The enormous spans referred to as galaxy groups and clusters contain hot gas, dark matter, and galaxies with supermassive black holes at their centers, creating a dynamic environment.
“The complex interactions between these components play a crucial role in shaping the life cycles of galaxies and driving the evolution of the groups and clusters themselves,” Gozaliasl explained. “By uncovering a more complete history of these cosmic structures, we can better understand how these processes have influenced the formation and growth of both massive galaxies and the largest structures in the universe.”
Instead of an even distribution, astronomers have found galaxies to cluster in dense regions due to the cosmic web, interconnections of filaments and walls that hold the clusters together. A small group may range in size from dozens of galaxies to single digits, yet single, unconnected galaxies remain a rarity. Larger clusters also form when gravity bands together hundreds or even thousands of galaxies as a single unit. The Milky Way lives in just a small galaxy group alongside Andromeda and a few dozen others that astronomers collectively dub “the Local Group.”
“Like humans, galaxies come together and make families,” explains Gozaliasl. “Groups and clusters are really important, because within them galaxies can interact and merge together, resulting in the transformation of galaxy structure and morphology. Studying these environments also helps us understand the role of dark matter, feedback from supermassive black holes, and the thermal history of the hot gas that fills the space between galaxies.”
Formation and Evolution
The new collection represents such a broad time period that scientists can better compare galaxy structures to understand galactic evolution. Additionally, this historical survey aids astronomers in studying how the brightest group galaxies (BGGs), large galaxies at the center of clusters, form as repeated mergers occur.
“When we look very deep into the universe, the galaxies have more irregular shapes and are forming many stars. Closer to our time, star formation is what we refer to as ‘quenched’– the galaxies have more symmetric structures, like elliptical or spiral galaxies. It’s really exciting to see the shapes changing over cosmic time. We can start to address so many questions about what happened in the universe and how galaxies evolved,” said Gozaliasl.
The paper “The COSMOS-Web Deep Galaxy Group Catalog up to z = 3.7” appeared on May 19, 2025, in Astronomy and Astrophysics.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.