COSMOS-z3.1-A Reveals How the Universe Built Its Largest Galaxy Systems

COSMOS-z3.1-A is a rare view of a proto-supercluster—an early ancestor of a future “cluster of clusters”—at a time when the universe was only about 2.1 billion years old. Astronomers estimate that the structure already contains roughly 5,000 times the mass of the Milky Way and could grow into a system more massive than the nearby Coma Cluster. That future outcome is an estimate based on the observed overdensity and models of structure growth, not a direct measurement of its eventual mass. 15

What astronomers found

The international team identified COSMOS-z3.1-A as the most distant known progenitor of a galaxy supercluster. It is more unusual than an individual protocluster: it is an emerging larger-scale complex expected to become a collection of massive galaxy clusters. 7

Rather than appearing as a single, settled sphere of galaxies, COSMOS-z3.1-A is clumpy and irregular. Its dense components sit in an exceptionally overdense region where cosmic-web filaments intersect, and filament-like extensions appear to connect to and feed the cores. 14

The research team’s descendant-mass estimates indicate that COSMOS-z3.1-A, along with COSMOS-z3.1-C, could reach a total mass above that of the Coma Cluster by redshift zero—the present-day universe. The paper reports estimated descendant masses of at least (10^{15.3}) solar masses for these extreme systems. 1

How ODIN turned a distant overdensity into a 3D map

The discovery began with the ODIN survey, which searches for high-redshift protoclusters using Lyman-alpha-emitting galaxies as tracers. ODIN’s observations in the COSMOS field used narrow-band imaging associated with the Dark Energy Camera (DECam) to identify galaxy concentrations at redshift 3.1. 335

A projected concentration on the sky is not enough to establish a physical structure. The team therefore added spectroscopic observations from the Dark Energy Spectroscopic Instrument (DESI), Gemini South’s GMOS instrument, and Keck II’s DEIMOS instrument. Those redshift measurements reconstructed the galaxies’ three-dimensional distribution and revealed the dense cores and their larger environment. 135

This approach also made the filaments visible through the distribution of the traced galaxies. In other words, the result is not simply a list of unusually distant galaxies; it is a map of a developing large-scale environment. 1

Why the structure supports bottom-up cosmic growth

Modern cosmology describes structure formation as hierarchical: smaller concentrations of matter form first, then merge and draw in additional material to make ever larger systems. COSMOS-z3.1-A is consistent with that picture because it contains multiple developing components embedded in a network of filaments, rather than a single mature cluster. 17

Its apparent location at the meeting points of cosmic-web filaments is important. Such filaments can channel galaxies and matter into dense nodes, helping the separate components of a proto-supercluster develop into a more massive system over cosmic time. The observation therefore offers a particularly early example of the intermediate stages between smaller protoclusters and the largest cluster systems seen nearby. 14

What this does—and does not—say about JWST’s “hidden stars” result

COSMOS-z3.1-A addresses the growth of large-scale environments: dark-matter-rich regions, galaxy groups, filaments, and protoclusters. A separate James Webb Space Telescope result concerns the stellar populations inside individual early galaxies.

In that JWST study, researchers examining nine massive early galaxies found evidence for many more faint, low-mass stars than commonly assumed. Because those stars contribute significant mass while producing little light, some inferred stellar masses could be three to four times higher than earlier estimates; the most extreme reported case approached a factor of four. 192027

The two findings are complementary, but neither is a complete explanation of the other. A filament-fed proto-supercluster offers a plausible setting for rapid galaxy assembly, while a bottom-heavy stellar population changes how much stellar mass may be hidden within some galaxies. Explaining both the fast buildup of dense environments and the detailed mix of stars within their galaxies remains a central challenge for models of the early universe. 11722

What comes next

The Vera C. Rubin Observatory has begun the Legacy Survey of Space and Time (LSST), and its first science-oriented LSST Camera data release included deep observations of the COSMOS field. 3336

Wide, deep imaging from Rubin can help identify more candidate high-redshift overdensities across large areas of sky. Spectroscopy will remain essential, however, because it supplies the precise redshifts needed to distinguish a real three-dimensional proto-supercluster from galaxies that only appear close together in a two-dimensional image. ODIN’s work on COSMOS-z3.1-A shows why combining wide imaging with broad and targeted spectroscopy is so powerful. 13