What an 8.5-Billion-Year-Old Space Jellyfish Is Telling Us — The Secret of the Most Distant Galaxy Webb Has Ever Caught

A research team led by Dr. Ian Roberts, a Banting Postdoctoral Fellow at the University of Waterloo in Canada, discovered jellyfish galaxy COSMOS2020-635829 at redshift z=1.156 in JWST data. This galaxy is seen as it was approximately 8.5 billion years ago, making it the most distant jellyfish galaxy ever observed. The results were published in The Astrophysical Journal (DOI: 10.3847/1538-4357/ae3824). A jellyfish galaxy gets its name from the tentacle-like gas tails that form when a galaxy moves rapidly through a galaxy cluster and the cluster's hot, dense gas pushes the galaxy's own gas backward. This process, known technically as ram-pressure stripping, causes dramatic changes to a galaxy's star formation activity and structure. The discovery was made in the COSMOS field, a deep-sky observation region studied by multiple telescopes.

The core shock of this discovery lies in timing. The prevailing view in astronomy was that ram-pressure stripping mainly occurred in the relatively recent universe, at low redshifts. But 8.5 billion years ago, the universe was only about 40% of its current age. The fact that cluster environments were already harsh enough to tear galaxies into jellyfish-like shapes at that epoch completely upends the existing theoretical timeline. The research team stated this finding suggests that galaxy cluster formation itself may have progressed faster than previously assumed. Dr. Roberts emphasized that this discovery provides rare insight into how galaxies were transformed in the early universe.

The third key finding relates to the origins of inactive or dead galaxies commonly found in modern galaxy clusters. The centers of present-day galaxy clusters are packed with red elliptical galaxies that no longer form stars. How and when did these galaxies stop making stars? This discovery suggests that since ram-pressure stripping was already operating 8.5 billion years ago, the environmental pressures that strip galaxies of gas and halt star formation have been at work since the early universe. In other words, the population of dead galaxies in modern clusters is the cumulative result of billions of years of environmental effects. This means the role of environment in galaxy evolution was far older and more fundamental than previously thought.

This discovery was made possible purely by JWST's overwhelming capabilities. During the Hubble era, jellyfish galaxies were observed, but almost exclusively in the relatively nearby universe at redshifts below 0.5. JWST's infrared observation capabilities enabled it to clearly capture the gas tail structure of a galaxy 8.5 billion light-years away. Compared to ESO 137-001, a jellyfish galaxy discovered in the Hubble era, the leap in observational reach is staggering. As JWST enters its fourth year of operations, discoveries like this about the early universe are expected to keep pouring in, with experts calling this the genuine beginning of astronomy's golden age.

This discovery demands a fundamental shift in how we understand the universe. Traditionally, astronomy focused on studying the properties of individual galaxies. But the existence of jellyfish galaxies dramatically demonstrates that galaxies do not exist in isolation and that their surrounding environment determines their fate. This is analogous to how ecology shows that a species' characteristics are determined by its habitat. The cosmic ecology perspective, viewing galaxies as members of an ecosystem rather than isolated objects, is becoming increasingly important, and this discovery has extended that perspective back 8.5 billion years. With the research team announcing plans to search for additional jellyfish galaxies in the COSMOS field, the revolution in galactic environmental science has only just begun.