The Jellyfish Galaxy JW39 hangs peacefully in this image taken by NASA/ESA’s Hubble Space Telescope. This galaxy is located 900 million light-years away in the constellation Veronica Coma and is one of several jellyfish galaxies Hubble has studied in the past two years.

Despite its carefree appearance, this jellyfish galaxy is drifting in an extremely hostile environment: the galaxy cluster. Galaxies in galaxy clusters are often distorted by the gravitational force of their larger neighbors, which can bend galaxies in different ways compared to their more isolated counterparts. If that’s not enough, the scorching hot plasma known as the intra-cluster medium also permeates the space between the galaxies in the cluster. Although this plasma is extremely sparse, galaxies moving through it almost feel like current-fighting swimmers, and this interaction could purge galaxies of star-forming gas.

This interaction between the intra-cluster medium and galaxies is called the rammer-pressure gap, and this is the process responsible for tightening the jellyfish galaxy’s tentacles. As JW39 moved through the cluster, the pressure of the intra-cluster environment forced gas and dust into long lanes of star formation now extending from the galaxy’s disk.

Astronomers using the Hubble Wide Field Camera 3 have studied these waves in detail, as they are a particularly extreme medium for star formation. Surprisingly, they found that star formation in the “tentacles” of jellyfish galaxies was not markedly different from star formation in the galactic disk.