In the Loktak protocluster, discovered at a distance corresponding to an era roughly 1.2 billion years after the Big Bang, galaxies in dense regions were already showing signs that their environment was influencing their growth and structure.
A research team from the National Astronomical Observatory of Japan utilized the Hyper Suprime-Cam on the Subaru Telescope in Hawaii to identify the protocluster, combined with infrared data from the James Webb Space Telescope (JWST) for a more detailed investigation. Their findings, published in May 2026, revealed that galaxies in the protocluster’s central regions were, on average, approximately 1.4 times larger in optical light—which traces mature star distribution—than galaxies in less dense "field" regions of the same era. subarutelescope.org
The objects exhibit a redshift of z ≃ 4.9, representing a lookback time of approximately 12.6 billion years. This is one of the earliest known protoclusters where scientists have successfully identified the influence of the environment on galactic structure. arxiv.org
Unlike modern clusters, where galaxies often show suppressed star formation, the differences here appear in their morphology: when observed in ultraviolet light (which tracks active star formation), no significant differences in size were found. This suggests that galaxies in dense environments built up their outer stellar structures more rapidly, while their central star-forming regions evolved in a similar fashion. subarutelescope.org
These observations indicate that the environmental influence on galaxy evolution began significantly earlier than previous models assumed, occurring as early as the first billion years of the universe's existence.
