A vast structure of galaxies was already assembling while the Universe was still very young.
Astronomers have named it COSMOS-z3.1-A. Its light traveled for about 11.7 billion years before reaching us. We therefore see it as it was just 2.1 billion years after the Big Bang. At that time, the Universe’s large-scale structures were still taking shape.
The 4-meter Víctor M. Blanco Telescope beneath its dome at Cerro Tololo, Chile, used in this discovery.
Credit: CTIO/NOIRLab/NSF/AURA. CC BY 4.0 License.
To understand what was discovered, we need to forget the image of a simple galaxy cluster. COSMOS-z3.1-A brings together several very dense regions that should eventually form clusters. Taken as a whole, it constitutes what astronomers call a proto-supercluster: the ancestor of a structure containing several galaxy clusters.
The researchers reconstructed this region in three dimensions. They combined galaxies identified in images with more precise measurements of their distances. These observations came notably from the Keck and Gemini telescopes, as well as the DESI instrument. This method reduces the risk of confusing galaxies that are truly near one another with objects that are merely aligned in the sky.
The resulting map reveals ten large concentrations of matter in COSMOS-z3.1-A. The structure is elongated and contains several extensions connecting its densest regions. These filaments could channel matter toward the regions where galaxies and future clusters grow.
Annotated view of the COSMOS-z3.1-A field, showing the positions of the proto-supercluster’s main galaxy concentrations.
Credit: CTIO/NOIRLab/DOE/NSF/AURA; processing by T.A. Rector, M. Zamani, D. de Martin. CC BY 4.0 License.
Its size and future mass are particularly remarkable. Calculations indicate that the whole structure could evolve into one exceeding 2 quadrillion solar masses today. That is several thousand times the mass of the Milky Way. The researchers also studied COSMOS-z3.1-C, another large concentration from the same period.
Why look so far back? Today’s superclusters are the result of billions of years of evolution under the influence of gravity. Finding their ancestors allows us to follow this construction much earlier. COSMOS-z3.1-A thus becomes the most distant confirmed proto-supercluster of its kind studied by this team.
The method should now be used to search for other very ancient structures. The ODIN program observes vast regions of the sky and still has other fields to analyze. Future data from the Vera C. Rubin Observatory will be able to complement these observations and help track cluster growth across a larger portion of cosmic history.