Webb Spots Most Distant Jellyfish Galaxy Ever Seen, Pushing Ram-Pressure Stripping Back 8.5 Billion Years
James Webb Space Telescope identifies galaxy COSMOS2020-635829 at redshift z=1.156, the most distant jellyfish galaxy known, revealing cluster environments were stripping galaxies far earlier than models predicted.
Overview
Astronomers at the University of Waterloo have identified the most distant jellyfish galaxy ever observed, using data from the James Webb Space Telescope (JWST). The galaxy, designated COSMOS2020-635829, sits at a redshift of z = 1.156 — meaning its light has traveled approximately 8.5 billion years to reach Earth. The findings, published in The Astrophysical Journal, push back the confirmed timeline for ram-pressure stripping to a period when the universe was less than half its current age.
What Jellyfish Galaxies Are
Jellyfish galaxies take their name from the long, tentacle-like filaments of gas and young stars that trail behind them as they move through galaxy clusters. The mechanism driving this distinctive morphology is ram-pressure stripping: as a galaxy races through the hot, diffuse gas that fills its host cluster, that gas acts as a headwind, sweeping material off the galactic disk and stretching it into luminous streamers. The stripped gas does not simply dissipate — it can continue to collapse and form new stars far outside the galaxy’s main body, producing the bright blue knots that make jellyfish galaxies visually striking.
Previously identified jellyfish galaxies had all been found at comparatively low redshifts, corresponding to the more recent universe. The question of how far back in cosmic time such stripping events could operate had remained open.
The JWST Observation
The discovery team, led by Dr. Ian Roberts, a Banting Postdoctoral Fellow at the Waterloo Centre for Astrophysics, analyzed JWST imaging from the COSMOS field — a deep survey region that has been observed extensively across multiple telescopes and wavelengths. The COSMOS field’s depth and JWST’s infrared sensitivity together allowed the team to identify the trailing gas filaments and the blue star-forming clumps embedded within them, features that ground-based and earlier space telescopes would have struggled to resolve at this distance.
Co-authors on the study include Michael L. Balogh, Visal Sok, Adam Muzzin, Michael J. Hudson, and Pascale Jablonka.
What the Discovery Implies
The significance of finding a jellyfish galaxy 8.5 billion years back is primarily what it reveals about the environments galaxies inhabited at that epoch. According to Dr. Roberts, the observation demonstrates two things: “cluster environments were already harsh enough to strip galaxies, and galaxy clusters may strongly alter galaxy properties earlier than expected.”
Many theoretical models had assumed that galaxy clusters at z ~ 1 were still in a relatively early phase of assembly — dense enough to host galaxies but not yet producing the extreme intracluster medium pressures required to strip them efficiently. COSMOS2020-635829 challenges that picture. If stripping was already operating at this cosmic epoch, it would mean that the transformation of spiral, star-forming galaxies into the passive, gas-poor ellipticals that dominate clusters today began substantially earlier than commonly modeled.
This has direct implications for understanding the large population of quenched — effectively dead — galaxies observed in galaxy clusters at lower redshifts. Ram-pressure stripping, which removes the raw material for star formation, is one of the leading mechanisms proposed to explain why cluster galaxies stop forming stars. The new discovery extends the window during which this process could have contributed to that transformation.
Context and Next Steps
The result joins a growing body of evidence from JWST that early galaxy clusters were more dynamically active than pre-Webb models anticipated. Earlier JWST surveys have identified mature, massive galaxies and overdense proto-cluster environments at redshifts well above z = 1, suggesting that structure formation in the universe proceeded rapidly.
For jellyfish galaxies specifically, the Waterloo team’s detection opens the possibility that systematic searches in existing and planned JWST deep-field data could uncover further examples at even greater distances, potentially tracing the onset of ram-pressure stripping across a broader span of cosmic time.