Somewhere in a patch of sky chosen precisely because it is boring, a galaxy is being torn apart. Not by collision or explosion, but by wind. The gas that threads through its spiral arms is being peeled away, flung into long trailing streamers that stretch tens of thousands of light years behind it, like tentacles. We are seeing this happen as it was 8.5 billion years ago, when the universe was barely a third of its current age.
The galaxy, with the unglamorous catalogue name COSMOS2020-635829, is the most distant jellyfish galaxy ever observed. Its discovery, reported by Ian Roberts at the University of Waterloo in Canada and colleagues, is forcing astronomers to reconsider how quickly the cosmos became a hostile place for galaxies.
Jellyfish galaxies get their name from what happens when they plunge through the hot, diffuse gas that fills the space between galaxies in a cluster. That intergalactic medium acts like a headwind, ram-pressure stripping the galaxy’s own gas out the back. Stars, being far denser, stay put. But the lighter gas gets swept into long tails, and sometimes, in those tails, new stars ignite. It is a process well documented in the nearby universe. Finding it at work this far back in cosmic time, though, is something else entirely.
Roberts and his team were sifting through data from the James Webb Space Telescope’s COSMOS-Web survey when they spotted it. “We were looking through a large amount of data from this well-studied region in the sky with the hopes of spotting jellyfish galaxies that haven’t been studied before,” Roberts said. “Early on in our search of the JWST data, we spotted a distant, undocumented jellyfish galaxy that sparked immediate interest.”
What JWST revealed was striking in its clarity. The main body of the galaxy looked perfectly normal, a symmetric disc of stars going about their business. But to the south, trailing behind like debris from a slow-motion car crash, sat four bright blue knots of light. These weren’t old stars ripped loose. They were new ones, born outside the galaxy altogether, condensing from stripped gas that had no business forming stars at all.
To confirm the knots were genuinely connected to the galaxy rather than unrelated objects in the background, the team turned to the Gemini North telescope in Hawaii. Using its integral field spectrograph, they mapped the faint glow of ionised oxygen gas across the region. The gas tail extended at least 20 kiloparsecs beyond the galaxy’s visible edge and, critically, showed a smooth velocity gradient linking it to the galaxy’s own rotation. That coherence was the clincher; this gas belonged to the galaxy, and it was being ripped away.
The tail sources turned out to be remarkably young, each one harbouring stellar populations less than 100 million years old, with masses around a hundred million times that of our sun. Individually, they account for roughly 1 to 2 per cent of the main galaxy’s stellar mass and only about half a per cent of its star formation rate. Modest contributions, perhaps. But the very fact that they exist this far back in time is what matters.
Before this finding, the number of confirmed jellyfish galaxies beyond a redshift of 1 was, by the paper’s own reckoning, probably zero. Fewer than ten candidates existed in the entire literature. The prevailing assumption held that galaxy clusters at these early epochs were still assembling, their environments not yet dense or hot enough to generate the brutal ram pressures needed to strip gas from a moving galaxy.
COSMOS2020-635829 suggests otherwise. “Cluster environments were already harsh enough to strip galaxies, and … galaxy clusters may strongly alter galaxy properties earlier than expected,” Roberts said. That has knock-on implications for one of astronomy’s most persistent puzzles: why so many galaxies in clusters today are red and dead, drained of the gas they need to form stars.
The conventional picture invokes a “slow-then-rapid” model. Galaxies first get starved of fresh gas as they fall into a cluster. Then ram-pressure stripping delivers the coup de grâce, sweeping away whatever fuel remains. If that stripping was already operating near Cosmic Noon, roughly the epoch when star formation across the universe peaked, it could mean the process of environmental killing started much earlier than we thought. Roberts and his team have requested more time on JWST to probe the galaxy further.
There is still a question mark hanging over the galaxy’s identity. The team cannot entirely rule out that it might be a collisional ring galaxy rather than a jellyfish. But the pattern of its ionised gas, peaking at the galaxy centre and fading steadily into the tail rather than concentrated along a ring, points strongly toward stripping. Whatever the final verdict, one thing seems clear: the young universe was not the gentle nursery we once imagined. It was already capable of skinning galaxies alive.
Study link: https://iopscience.iop.org/article/10.3847/1538-4357/ae3824
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