COSMOS-z3.1-A: Ancestor of a 5,000-Milky-Way Supercluster
Astronomers have found COSMOS-z3.1-A, the most distant known ancestor of a galaxy supercluster, with a mass of about 5,000 Milky Ways, using the ODIN survey and the Blanco telescope in Chile.
An international team of astronomers has identified the most distant known ancestor of a galaxy supercluster, a structure designated COSMOS-z3.1-A, at a redshift of 3.1. The discovery was published on September 8, 2026, in The Astrophysical Journal, under DOI 10.3847/1538-4357/ae8091. The study’s lead author is Vandana Ramakrishnan, a doctoral student at Purdue University at the time of the research, and the structure’s total mass is estimated at roughly 5,000 times the mass of the Milky Way.
Galaxy clusters are the most massive gravitationally bound structures in the universe, made up of hundreds to thousands of galaxies spread across millions of light-years. The clusters astronomers observe in the nearby universe are mature systems that grew out of protoclusters, immense and loosely bound groups of galaxies still in the process of merging. To understand how today’s clusters formed, researchers look for protoclusters from an era when the universe was far younger, and that search is what led the team to COSMOS-z3.1-A.
The data come from the ODIN survey (One-hundred-deg2 DECam Imaging in Narrowbands), led by Kyoung-Soo Lee of Purdue University and Eric Gawiser of Rutgers University. The observations were carried out with the Dark Energy Camera, mounted on the 4-meter Victor M. Blanco Telescope in Chile, over more than 100 nights spread across three years. The team used the survey to identify 150 distant protoclusters that formed when the universe was between 1 and 3 billion years old, then focused on two structures with a pronounced excess of galaxy density: COSMOS-z3.1-A and COSMOS-z3.1-C.
Sources disagree on how old the universe was when the light from COSMOS-z3.1-A began its journey to Earth. Phys.org puts the figure at 2.1 billion years, while a Rutgers University statement gives 2.8 billion years. Rutgers also describes the same distance as roughly 12 billion light-years.
How the Third Dimension Was Confirmed
The first images from the ODIN survey produced only a two-dimensional map of galaxies on the sky, not enough to tell whether they truly belonged to the same structure or simply overlapped by chance in projection. To settle the question, Ramakrishnan, together with fellow doctoral researchers Byeong Mun of the Korea Astronomy and Space Science Institute (KASI) and Nicole Firestone of Rutgers, carried out follow-up observations with spectrographs, instruments that use the properties of light to measure an object’s distance. Most of the spectra came from the DESI instrument, capable of measuring the distances of up to 5,000 galaxies at once, mounted on the 4-meter Nicholas U. Mayall Telescope in Arizona. Additional data came from the GMOS spectrograph on the Gemini South telescope and the DEIMOS spectrograph on the Keck II telescope.
“COSMOS-z3.1-A represents the most extreme, densest regions of the universe,” said Ramakrishnan, the study’s lead author, in a statement released by the research team to announce the paper.
The three-dimensional maps showed that ten galaxy clusters are forming inside COSMOS-z3.1-A. Based on the models the team used, an object like this appears less often than once in every 10,000 galaxy clusters, making it one of the rarest known structures of its kind. The arrangement of galaxies is clumpy and irregular, and the structure sits at the intersection of several filaments of the cosmic web, which the team cites as support for the model in which larger structures are built from the bottom up, as smaller ones merge into ever-larger wholes.
“When we look at galaxy clusters in the nearby universe, we’re looking at the finished product. This distant structure takes us back to a much earlier stage,” said Eric Gawiser, a professor in the Department of Physics and Astronomy at Rutgers University, in a statement from the university.
Alongside Gawiser, the Rutgers team included doctoral student Nicole Firestone and recent graduate Govind Ramgopal, who helped analyze the additional measurements.
What This Discovery Does Not Prove
The paper does not show what COSMOS-z3.1-A will actually look like once it matures. The estimate that it will surpass the mass of the Coma Cluster, the largest known cluster in the nearby universe, comes from models applied to a snapshot of the structure as it stands today, not from direct observation of its later evolution. The estimate that an object like this appears less than once in every 10,000 clusters likewise comes from comparison with a single simulation used to test the team’s methodology, not from independent verification across multiple simulations. The gap between the ages cited by the two sources, Phys.org and Rutgers University, shows that even secondary reports on the same paper don’t always carry identical figures, so readers looking for a precise value should consult the paper itself in The Astrophysical Journal. The team says the Rubin Observatory, through its Legacy Survey of Space and Time, will expand this kind of research over the next decade by surveying the entire southern sky every few nights.
