Have you ever looked up at the night sky and imagined finding something completely new, something that looks like it belongs in the deepest oceans? Here at FreeAstroScience.com, we live for these moments of discovery. Recently, astronomers using the incredible James Webb Space Telescope (JWST) did just that. They spotted a "jellyfish" galaxy, a breathtaking and violent spectacle happening billions of light-years away.
This discovery, brought to you by us at FreeAstroScience.com, where we explain complex scientific principles in simple terms, is more than just a pretty picture. We invite you, our most valued reader, to join us as we explore what this cosmic jellyfish is and what it tells us about the universe's past. Stick with us to the end for a truly deep understanding of this magnificent finding.
Thumbnail images of COSMOS2020-635829 for the 4 JWST filters used in the study. The RGB image on the right 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: arXiv (2025). DOI: 10.48550/arxiv.2506.14117
What Exactly Is a "Jellyfish" Galaxy?
When we talk about a "jellyfish" in space, we're not talking about a biological creature. We're describing a galaxy undergoing one of the most dramatic transformations imaginable. These galaxies get their name from their appearance: long, trailing "tentacles" of gas, dust, and newborn stars streaming from one side of the main galactic disk.
But what causes this? The process is a powerful force called ram pressure stripping (RPS).
Imagine a swimmer diving into a pool. The water pushes back against them, trying to slow them down. For a galaxy, the "pool" is a super-hot, dense cloud of gas that fills the space between galaxies in a cluster. This is called the intracluster medium (ICM). As the galaxy plummets through this medium at incredible speeds, the ICM acts like a cosmic wind, stripping away the galaxy's own gas and dust.
This process is both destructive and creative. While it robs the galaxy of its fuel for future star formation, the stripped gas itself can be compressed, triggering furious bursts of new star birth within the trailing tentacles. The result is a beautiful, transient object that looks remarkably like a jellyfish drifting through the cosmic sea.
How Was COSMOS2020-635829 Discovered?
This latest cosmic creature was spotted by a team of astronomers from Canada and Switzerland, led by Ian D. Roberts of the University of Waterloo. Their findings, published on June 17, 2025, relied on the unparalleled power of the James Webb Space Telescope. Using JWST's high-resolution imaging from the COSMOS-Web Survey, they identified a galaxy designated COSMOS2020-635829.
What they saw was the classic sign of a jellyfish candidate:
- A relatively symmetric, undisturbed main stellar disk.
- A one-sided tail made of bright, blue, compact knots.
These knots were a dead giveaway. They were bluer than the main galaxy, suggesting they were packed with young, hot stars—exactly what you'd expect from star formation triggered by ram pressure. The galaxy itself is a heavyweight, with a stellar mass of about 10 billion times that of our Sun and a star formation rate of around 100 solar masses per year, making it a "starburst" galaxy.
This galaxy isn't alone. It lives in a crowded neighborhood, a (proto)cluster of galaxies, which provides the dense intracluster medium needed for ram pressure stripping to occur.
What Makes This Jellyfish So Special?
Finding a jellyfish galaxy is always exciting, but COSMOS2020-635829 is a record-breaker. It's special for two key reasons: its confirmed gaseous tail and its incredible distance from us.
A Tail of Gas and Newborn Stars
Seeing knots of stars is one thing, but to confirm the jellyfish hypothesis, scientists needed a smoking gun. They had to prove that those knots were embedded in a tail of gas physically connected to the galaxy. Using the Gemini North telescope in Hawai'i, the team pointed the GMOS instrument at the galaxy to hunt for the faint glow of an ionized gas tail.
And they found it.
The observations revealed a tail of ionized gas extending about 20,000 light-years from the galaxy's disk, perfectly aligned with the stellar knots seen by JWST. Even better, by measuring the velocity of this gas, they showed it was moving in a coherent way, kinematically linked to the main galaxy's rotation. This confirmed that the gas and the star-forming knots were not a chance alignment but were truly stripped from COSMOS2020-635829.
The properties of these star-forming knots in the tail are mind-boggling:
- Stellar Masses: Around 100 million times the mass of our Sun.
- Star-Formation Rates: Churning out 0.1 to 1 solar masses worth of stars per year.
- Age: The stars are incredibly young, having formed within the last 100 million years.
This is direct evidence of star birth happening on a massive scale outside the main body of a galaxy.
Why Does Its High Redshift Matter?
Perhaps the most crucial fact about COSMOS2020-635829 is its redshift, measured at z = 1.156. In cosmology, redshift is our time machine. Because light takes time to travel, looking at distant objects is like looking into the past. A redshift of 1.156 means we are seeing this galaxy as it was when the universe was only about 5.7 billion years old—a period nicknamed "Cosmic Noon" when star formation across the universe was at its absolute peak.
This discovery is monumental because it is:
- The highest-redshift example of a ram-pressure stripped ionized gas tail ever confirmed.
- The highest-redshift example of extra-planar star formation (star birth in a stripped tail) ever confirmed.
It proves that ram pressure stripping was a significant force shaping galaxies much earlier in the universe's history than we had directly observed before. This process, known as "environmental quenching," is a key reason why galaxies in dense clusters eventually stop forming stars and "die." Seeing it in action near Cosmic Noon gives us a vital piece of the puzzle of galaxy evolution.
Conclusion
So, we've journeyed far across space and time to meet COSMOS2020-635829. It's not just a beautiful, strange-looking galaxy. It's a snapshot of a violent, transformative process, caught in the act by the powerful eye of the JWST. This single discovery pushes our understanding of how galaxies evolve in the universe's most crowded and hostile environments.
What will happen to those star-forming knots in the tail? Will they survive as tiny, independent "dwarf" galaxies, forever separated from their parent? Or will they disperse, seeding the space between galaxies with intracluster light? These are the fascinating questions that drive us forward.
This article was written specifically for you by FreeAstroScience.com. We believe you should never turn off your mind and must keep it active at all times, because the sleep of reason breeds monsters. Come back soon to keep exploring the cosmos with us and feed your curiosity.
More information: Ian D. Roberts et al, JWST Reveals a Likely Jellyfish Galaxy at z=1.156, arXiv (2025). DOI: 10.48550/arxiv.2506.14117
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