Astronomers have discovered a remarkably tiny group of six young galaxies just 1.2 billion years after the Big Bang. This may be a rare glimpse of how some of the universe’s largest galaxies formed. The paper outlining the findings was submitted to the arXiv preprint server on July 13.
Chaotic patches
The widely accepted cosmological model of the universe known as the Lambda Cold Dark Matter Model suggests that galaxies primarily form hierarchically through mergers. That means they grow piece by piece, as smaller galaxies merge over billions of years. In this context, dense regions in the early universe serve as natural laboratories to test this idea.
These dense patches, known as proto-clusters and proto-groups, are young clusters of galaxies packed into a region just tens of thousands of light-years across and represent an especially brief and extreme stage in galaxy evolution. Spotting them requires telescopes sensitive enough to detect faint, low-mass galaxies at extreme distances, and precise enough to confirm that the galaxies are truly bound together.
In this new study, the team led by Ronaldo Laishram of the National Astronomical Observatory of Japan studied one such compact galaxy proto-group at z = 4.97 called SCGG-z5. The group consists of six galaxy components, all spectroscopically confirmed—meaning the distance to each is estimated by identifying specific spectral lines in its light, rather than estimated from color and brightness alone.
Unsettled system
Using deep JWST imaging and slitless spectroscopy from the SAPPHIRES survey, the team found the group’s six galaxies confined to a small patch of sky, just 16,000 parsecs (about 52,000 light-years) across—roughly half the diameter of the Milky Way.
Measuring the stellar mass and individual masses, the team found that three galaxies sit at or above the typical star-formation rate for their epoch, with one forming stars notably faster than expected.
All six galaxies show disturbed and irregular shapes, aligning with the expectations from ongoing gravitational interactions or a merger scenario. Star-formation maps hinted that gas may be funneling inward toward the cores of some members. This may suggest inside-out growth in some of the members. The most massive galaxy, however, shows a tentative opposite pattern: quieter in the center, more active in the outskirts.
Researchers suggest that these differing properties among the members indicate that “the group environment is already differentiating the evolutionary trajectories of its members well before the system coalesces.”
When they studied the velocity spread among the six galaxies, they found that this is a dynamically “busy,” unsettled system, not a calm one. The galaxies are moving fast enough relative to each other that they could complete multiple close passes within roughly 14 million years.
All six unite
Based on comparisons with the EAGLE cosmological simulation, which has modeled similarly compact galaxy groups, the team expects SCGG-z5’s six galaxies to fully merge into a single system by around z ~ 3–4, which is roughly 400 million years from the time of observation. From there, the newly merged galaxy is predicted to keep growing, reaching a stellar mass of 100 billion suns by z ~ 1, potentially marking the early formation of a future cluster’s central, brightest galaxy.
The researchers suggest the whole assembly process, starting from the onset of star formation to final coalescence, spans about 800 million years. That would mean SCGG-z5 has been caught mid-journey for a brief cosmic moment, in the act of becoming something much larger.
As the team put it, SCGG-z5 offers “direct observational constraints on early group assembly” in the universe’s first billion years. They call for future JWST spectroscopy of gas motion and composition combined with ALMA observations of cold gas to further test whether tidal interactions are truly driving the differences seen between galaxies.
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Publication details
Ronaldo Laishram et al, A Compact Proto-group at z∼5: A Massive Galaxy Caught in Formation, arXiv (2026). DOI: 10.48550/arxiv.2607.11182
Journal information:
arXiv
Key concepts
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Astronomers catch massive galaxy assembling piece by piece 1.2 billion years after Big Bang (2026, July 24)
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