What Are Jellyfish Galaxies and How Do They Form?

Galaxies With Tentacles

Scattered across the universe's densest neighborhoods, some galaxies look nothing like the neat spirals or smooth ellipticals found in textbooks. Instead, they trail long, luminous streamers of gas and young stars — features so striking that astronomers call them jellyfish galaxies. The resemblance to their oceanic namesakes is uncanny: a compact body at the front and wispy tentacles streaming behind, sometimes stretching more than 300,000 light-years into the void.

How Ram-Pressure Stripping Works

The key to understanding jellyfish galaxies lies in a process called ram-pressure stripping. Galaxy clusters — gravitationally bound collections of hundreds or thousands of galaxies — are filled with extremely hot, thin gas known as the intracluster medium (ICM). When an individual galaxy plunges into a cluster at speeds that can exceed 1,000 kilometers per second, the ICM acts like a powerful headwind.

If that headwind exerts enough pressure to overcome the galaxy's own gravitational pull on its gas, it begins to peel material away from the outer disk. Cold hydrogen, dust, and even molecular clouds are pushed outward, forming the long trailing tails that define a jellyfish galaxy. The force depends on two factors: the density of the surrounding cluster gas and the square of the galaxy's velocity through it.

The stripping process is not instantaneous. Simulations using the IllustrisTNG model show that ram-pressure stripping can reshape a galaxy over timescales of 1.5 to 8 billion years, depending on the mass of the host cluster and the amount of cold gas the galaxy carried when it arrived.

Stars Born in the Tentacles

One of the most surprising findings is that the stripped gas does not simply disperse. As it streams away from the galaxy, it can cool and collapse to form new clumps of stars inside the tails themselves. These bright blue knots of star formation — visible in Hubble and James Webb Space Telescope images — represent stellar nurseries that exist far from any galactic disk.

The jellyfish galaxy JO206, one of the best-studied examples, hosts a tail of ionized gas stretching at least 300,000 light-years. Research published in the Monthly Notices of the Royal Astronomical Society found that JO206 forms stars at a significantly higher rate than comparable spiral galaxies, with its hydrogen gas being consumed roughly four times faster than normal.

Why Jellyfish Galaxies Matter

Jellyfish galaxies offer astronomers a real-time window into galaxy evolution. Dense cluster environments are known to suppress star formation over time — a phenomenon called "quenching." Jellyfish galaxies capture the moment when that transformation begins, helping scientists understand why galaxies in crowded regions tend to be redder, older, and gas-poor compared with those in the field.

Research has also revealed a connection between ram-pressure stripping and supermassive black hole activity. Jellyfish galaxies are more likely to host active galactic nuclei (AGN) than non-stripped galaxies of similar mass, suggesting that the same forces peeling gas outward may also funnel some material inward toward the central black hole.

Pushing the Frontier Further Back in Time

In early 2026, a University of Waterloo team announced the discovery of the most distant jellyfish galaxy ever observed, seen as it appeared 8.5 billion years ago. Spotted by JWST and catalogued as COSMOS2020-635829, it reveals that ram-pressure stripping was already at work when the universe was less than half its current age — far earlier than many models predicted.

That finding matters because it shifts the timeline of galaxy transformation. If environmental processes were already sculpting galaxies billions of years ago, models of cosmic evolution need to account for a much earlier onset of cluster-driven change.

From the tentacle-trailing JO206 to the record-breaking COSMOS2020-635829, jellyfish galaxies continue to challenge and refine our understanding of how the universe's grand structures shape the galaxies within them.