A jellyfish galaxy is at the top-right, with star-forming tendrils streaming away towards the bottom-left (Image: NASA, ESA, and the Hubble Heritage Team (STScI/AURA))

Object: Jellyfish galaxies

Habitat: Crowded clusters of cosmic blobs

The large spiral galaxy was tired of being alone. It had been ages since its last relationship with another galaxy, and they had enjoyed a long, slow dance in each other’s orbit. When they finally merged, the spiral felt like it had done some serious growing, and it wanted even more social interaction. But it turns out that spiral galaxies need to change in a much more fundamental way if they want to move in with groups of friends.

A hunt through images from the Hubble Space Telescope has turned up half a dozen spiral galaxies that are being ripped apart and remade into jellyfish – with blobby bodies and glowing tendrils of stars – as they move towards joining galaxy clusters. It is thought that this process ultimately turns spirals into elliptical-shaped galaxies. That means the discovery, which more than doubles the number of known jellyfish galaxies, should help researchers better understand why such “ellipticals” are more common in clusters than their spiral cousins.


Galaxies can live in relative isolation, like our Milky Way, and only change shape significantly if they happen to collide with another galaxy. But in denser parts of the universe, gravity pulls galaxies together into enormous clusters. Previously, researchers had noticed that these clusters contain many more elliptical galaxies than spirals, hinting that newcomer spirals were somehow being transformed.

Blasted gas

Harald Ebeling at the University of Hawaii in Honolulu and his colleagues think jellyfish galaxies capture the process in action. “We see them turning one into the other,” says Ebeling. “They’re caught in the act.”

The space between the galaxies in a cluster is laced with dark matter and gas, which reaches searing hot temperatures due to the pressure from its crowded surroundings. That means when new galaxies join a cluster, they can’t just slip in quietly, says Ebeling.

Hot gas in the cluster smacks into cold gas within a new arrival, blasting the cold gas outwards in long streams. The stripped body of the galaxy settles into a blobby shape, while cold gas in the tendrils compresses enough to ignite new stars.

Ebeling and colleagues unexpectedly caught their first jellyfish in late 2005 and have been hunting for more extreme examples in Hubble images since then. Until recently, though, the transformation had been spotted only a few times in relatively nearby clusters. That’s probably because the change is over too fast, says Alastair Edge at the University of Durham, UK. And once a spiral galaxy has been stripped if its cold gas, it won’t experience another drastic transformation.

Orphan stars

“You’ve defused the bomb,” says Edge, “and it will settle down to be much more sedate and much more like all the other galaxies we see. This is a very rough period that it undergoes, and observationally it’s quite hard to see.” Now Ebeling’s team has caught six obvious jellyfish, clearly visible in Hubble images, which should help them better understand this unique galactic morphing.

“These images are stunning – you can see what’s happening,” says Ebeling. “You take a spiral galaxy, and it’s getting completely annihilated and destroyed by the gas that it’s running into.”

Studying jellyfish in detail could help solve another mysterious feature of galaxy clusters: why they contain relatively young “orphan” stars that do not belong to any particular galaxy. The gas inside clusters is too hot to collapse into new stars, so the stars must come from outside – possibly from the tentacles of jellyfish galaxies.

Journal reference: Astrophysical Journal Letters, DOI: 10.1088/2041-8205/781/2/L40