Six galaxies caught mid-merger 1.2 billion years after the Big Bang

The James Webb Space Telescope has captured a remarkable snapshot of cosmic history: six young galaxies squeezed into a patch of sky just 52,000 light-years across — roughly half the width of the Milky Way — all bound together in an active merger only 1.2 billion years after the Big Bang. If current cosmological models hold, in some 400 million years they will finish coalescing into a single massive galaxy, likely destined to become the brightest central galaxy of a future cluster.

The result, led by Ronaldo Laishram at the National Astronomical Observatory of Japan, was posted to arXiv on July 13, 2026, and offers what the team describes as some of the first direct observational constraints on how the universe’s largest galaxies assemble.

A compact proto-group at z ≈ 5

The system, catalogued as SCGG-z5, contains six galaxy components whose distances were confirmed spectroscopically — not merely inferred from color and brightness. That distinction matters: photometric redshift estimates at these distances are notoriously ambiguous, and confirming that all six lie at essentially the same redshift is what makes SCGG-z5 a genuine bound proto-group rather than a chance projection along the line of sight.

The observations combine deep JWST imaging with slitless spectroscopy from the SAPPHIRES survey. All six galaxies fit within a region only 16,000 parsecs across, and their kinematic behaviour points to a dynamically unsettled configuration: close passes can occur on timescales of roughly 14 million years.

Growth already differentiated

The individual members show distinct evolutionary paths. Three of the six galaxies exhibit star-formation rates at or above the typical value for their epoch, and one is forming stars notably faster than expected. All six display disturbed, irregular morphologies — consistent with strong tidal interactions and ongoing minor mergers.

Star-formation maps hint that some members are actively funneling gas inward toward their cores — the classic «inside-out» growth signature. Curiously, the most massive galaxy in the group shows the opposite pattern: a quieter core with more activity in the outskirts. The team notes that the group environment is already sculpting the members’ futures well before the system finally coalesces.

A future brightest cluster galaxy

Comparisons with the EAGLE cosmological simulation, which has modeled analogous compact groups, suggest SCGG-z5’s six galaxies should merge into a single system by z ≈ 3–4 — around 400 million years after the epoch of observation. The merged galaxy is then expected to grow to some 100 billion solar masses by z ≈ 1, a mass scale consistent with the central, brightest galaxies seen in today’s clusters.

In total, the researchers estimate the full assembly process — from the onset of star formation to final coalescence — spans roughly 800 million years. SCGG-z5 therefore represents a brief cosmic snapshot: caught mid-journey, in the act of becoming something much larger.

Future JWST spectroscopy of gas kinematics and composition, together with ALMA observations of the cold gas reservoirs, will be needed to establish whether tidal interactions are indeed the primary driver of the members’ diverging evolutionary paths.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: Ronaldo Laishram et al., A Compact Proto-group at z∼5: A Massive Galaxy Caught in Formation, arXiv:2607.11182 (2026). DOI: 10.48550/arXiv.2607.11182