Jellyfish appear to swim through the heavens just as they do through the seas. In recent years astronomers have spotted spiral galaxies that resemble the exotic creatures, trailing blue tendrils of gas and young stars. Now a search for more of these intriguing galaxies has yielded insights into their origin.

To locate celestial jellyfish, astronomers Conor McPartland and Harald Ebeling of the University of Hawaii at Manoa and their colleagues searched within 63 galaxy clusters, which harbor numerous large galaxies embedded in torrid gas. The team already knew that “jellyfish” arise when a luckless spiral galaxy falls into a galaxy cluster and the hot gas there strips away the spiral's gas, yielding streamers that spawn new stars. The brightest young stars shine blue, lending color to the jellyfish's tentacles. In total, nine previously undiscovered jellyfish galaxies were found.

All was not as expected, however. “We realized there's something funny here,” Ebeling says. “These galaxies are not moving toward the centers of the clusters.” Because of each cluster's gravitational pull, the jellyfish should have been heading into the cluster cores—a direction signaled by the trailing tentacles. Yet the galaxies were moving every which way. Similarly, the jellies' whereabouts were strange: they all dwelled in the cluster outskirts.

These observations suggest jellyfish creation also requires a cluster-cluster collision, during which speeding galaxies from one cluster smash through the hot gas of the other. In the ensuing chaos, galaxies would dart through space in all directions, as seen in the new data set. The research was published in January in Monthly Notices of the Royal Astronomical Society.

To confirm this idea, the researchers now plan to examine the gas in clusters harboring jellyfish galaxies. Cluster gas is so hot that it emits x-rays, but in the collision scenario, the gas in jellyfish-bearing clusters should be especially hot where the clusters smack into one another. If x-ray observations verify this scenario, galactic jellyfish must be victims of violence. Born in the aftermath of enormous collisions, they are destined to lose all their gas and metamorphose into elliptical galaxies, bland objects that lack gas and thus the beauty of star-spawning spiral arms.