Astronomers have taken a patch of distant sky and turned it into a 3D map, revealing a colossal system with about 5,000 times more mass than the Milky Way in the early universe.
In previous space pictures, this area looked like a flat painted ceiling. Galaxies appeared close together simply because they lined up from our point of view, even if some were separated by billions of light‑years in depth.
Because of that optical illusion, astronomers couldn’t say for sure whether they were looking at one gigantic galaxy cluster under construction or a corridor where many clusters were in development.
The 3D map answered that. By measuring how far away the galaxies are, scientists watched that view transform into a chain of clumps. That new way of looking at the data showed this was not a construction zone for a regular galaxy cluster but a supercluster — a cluster of clusters. Rather than one heap of galaxies, they were looking at cities of clusters.
“I think the distance and sheer scale of the structures we are studying is really extraordinary. These are structures containing hundreds to thousands of galaxies,” said Vandana Ramakrishnan, a graduate student at Purdue University in Indiana and lead author of the study, in a statement. “It brings home the vastness of the cosmos.”
The main system mapped is called COSMOS‑Z3.1‑A, and it’s the earliest supercluster ever found. A second, smaller one, COSMOS‑Z3.1‑C, sits nearby in the same field. Both are so distant that we see them as they were when the universe was only about 2 or 3 billion years old, less than a quarter of its current age.
Early surveys could only conclude that these were crowded locations for young galaxies. The clues came from a project called ODIN, which used the Dark Energy Camera on the U.S. National Science Foundation’s Blanco 4‑meter telescope in Chile. In those images, the combined region stood out as a place where far‑off galaxies seemed tightly packed together.
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The team turned to large telescopes in Chile, Hawaii, and Arizona, using instruments that spread each galaxy’s light into a detailed rainbow. Because the universe is expanding, light from distant galaxies is stretched toward redder colors. Measuring that stretch reveals how long the light has traveled and how far away a galaxy lies.
In three dimensions, COSMOS‑Z3.1‑A broke down into 10 dense clumps of galaxies. COSMOS‑Z3.1‑C showed a similar pattern on a smaller scale, with about four. Each clump is massive and crowded enough to grow into its own galaxy cluster.
Scientists say the new 3D view makes it easier to test the basic story of how the universe built its largest architecture. In computer models, groups of galaxies form first. They slide along highways, or filaments, under gravity, piling into chains of clumps. Over time, they merge into rounder, more settled clusters and superclusters. The results of the study were published in The Astrophysical Journal.
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The 3D map also shows where these growing giants fit into the universe’s larger cosmic web. Matter gathers into long highways that meet at intersections. Those hubs are where the clusters and superclusters tend to collect.
The Dark Energy Camera on the Víctor M. Blanco 4-meter Telescope in Chile helped astronomers find two unusually dense superclusters in the early universe.
Credit: Nicole Firestone
“When we look at galaxy clusters in the nearby universe, we are seeing the finished product,” said Eric Gawiser, a Rutgers University physics and astronomy professor, in a statement. But “This distant structure takes us back to a much earlier stage when the individual pieces were still coming together. It allows us to study how the universe built structures on its largest scales.”
Once the galaxies were arranged in 3D, the researchers could estimate how much mass they have. They found both systems should eventually become more massive than the Coma Cluster, one of the largest known clusters in our area. In particular, COSMOS‑Z3.1‑A is head and shoulders above other heavyweights.
“We think there should be fewer than one such object for every 10,000 galaxy clusters,” Ramakrishnan said.
The new Vera C. Rubin Observatory in Chile should make it possible to study more superclusters like this. Eventually, researchers hope to have enough data to compare how these systems look at different ages and track how galaxies evolve into dense cluster cores.
