A cosmic headwind
In the vast emptiness between galaxies within a cluster, space is not empty. It is filled with a superheated, thin gas called the intracluster medium (ICM). This plasma, heated to millions of degrees, is a formidable obstacle for any galaxy moving through it. When a galaxy plunges through a cluster at speeds up to 4.5 million miles per hour (nearly 2,000 km/s), the ICM is a powerful headwind. This "wind" exerts a pressure, known as ram pressure, on the galaxy's own interstellar medium—the gas and dust between its stars.
If the ram pressure is stronger than the galaxy's gravitational pull on its own gas, that gas is effectively blown out of the galaxy, trailing behind in long streamers. This process is called ram pressure stripping. The galaxy's stars, bound by stronger gravity, are mostly unaffected. The result is a visually spectacular object: a "jellyfish galaxy," with a relatively intact stellar disk and long, trailing tentacles of gas stretching for tens of thousands of light-years. The galaxy ESO 137-001, for instance, has a tail extending across 260,000 light-years of space. This stripping process is a factor in galactic evolution, as it removes the raw fuel needed for future star formation, effectively "quenching" the galaxy over time.