The most distant known example of what astronomers call a “jellyfish galaxy”—an uncommon type of galaxy that is undergoing gas stripping at an accelerated rate—has been spotted by a team using NASA’s James Webb Space Telescope.
The rare observation provides a glimpse into the dynamics of galactic evolution as they occurred in the far more crowded conditions that prevailed in the early universe.
The newly identified galaxy, which has been designated COSMOS2020-635829, exists more than 8 billion light-years away from Earth at a redshift of 1.156, according to a new paper detailing the discovery that appeared on the arXiv.org preprint server.
The international team behind the new research reports that high-resolution imagery obtained by the Webb Telescope has revealed a one-sided tail of what appear to be very bright, star-forming knots that can be seen trailing away from the galaxy’s disk, forming the jellyfish’s “tentacles.”
Spectroscopic data obtained by Webb confirmed that these features are linked to an ionized gas tail, which was likely formed as a result of the galaxy’s movement through dense intergalactic material in a proto-cluster environment.
The team reports that its discovery provides rare direct evidence of the early occurrence of ram-pressure stripping in the universe, a powerful force that can shut down stellar formation.
Combined Imagery Reveals a Cosmic Jellyfish
High-resolution imagery obtained by Webb was combined with existing Hubble data, as well as imagery from the Subaru and Gemini observatories, revealing COSMOS2020-635829 as a galaxy with a symmetrical stellar disk and possessing a striking plume of star-forming regions, which the team estimated to extend approximately 20 kiloparsecs beyond the primary galactic structure.
Moreover, the star-forming knots in the structure’s tail are believed to be less than 100 million years old, making them exceptionally young, each possessing stellar masses close to 100 million times that of our Sun. Each of these knots displays rates of star formation at around 0.3 solar masses each year.
Additional spectroscopic data obtained by Gemini’s GMOS instrument revealed that the ionized gas in the tail is linked to the main galaxy kinematically—that is, its connection with the greater galactic body is related to the galaxy’s motion. This observation helps confirm its physical association, as opposed to merely representing a chance alignment.
A crucial finding by the team is that the ionized tail is believed to represent the highest-redshift ever confirmed in relation to such a structure produced by ram-pressure stripping. This process occurs when gas is swept from a galaxy as it plunges into hot, extremely dense regions of plasma located within a galaxy cluster, leaving behind trails of stripped gas where new stars can form.
A Dense Galactic Habitat
Webb’s recent imagery of COSMOS2020-635829 reveals that it resides in an exceptionally dense environment, which is believed to be part of an emerging galaxy cluster with a total mass of about 10^14 solar masses based on X-ray observations.
is a combination of the JWST F444W (red channel), the F277W (green channel), and F115W+F150W (blue channel). The
dashed circles mark the four extra-planar sources that are identified in the tail of COSMOS2020-635829 (Credit: Credit: arXiv (2025). DOI: 10.48550/arxiv.2506.14117).
The research team argues that the conditions that exist in this proto-cluster are probably sufficient to produce the ram pressure required to strip gas from this unique “jellyfish” galaxy at such an early cosmic epoch.
Additionally, the detection appears to suggest that environmental forces were already shaping galaxy evolution during an era when the universe was less than 5 billion years old.
Quenching Galaxies and Light Accumulation
The team’s discovery also sheds new light on one of the unique processes of galactic formation, involving how they are quenched, a process that effectively halts their star formation. This also allowed the team to gauge possible mechanisms through which intracluster light might be built up over time.
The star-forming knots present within COSMOS2020-635829’s tail may either fall back into the galaxy or they might contribute to the diffusion of light within the cluster. Another possibility altogether is that they may survive as compact stellar systems.
Fundamentally, the team attributes their findings to “the powerful combination of depth and angular resolution afforded by the James Webb Space Telescope.”
“This permits spatially resolved analyses in order to directly constrain the quenching mechanisms at work in the high-redshift Universe,” the team writes.
Going forward, additional multi-wavelength observations made with Webb and other advanced observatories, especially those detections involving molecular gas and H-alpha emission, will likely play a crucial role in helping astronomers understand how these processes occur.
The team’s new paper, “JWST Reveals a Likely Jellyfish Galaxy at z = 1.156,” appeared on the arXiv pre-print server on June 17, 2025.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.