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.
Unexpected birth in the tentacles
One of the most surprising discoveries about jellyfish galaxies is that their violent formation process also creates new life. The stripped gas in the tentacles, though torn away, is also compressed by the ram pressure. This compression can be enough to trigger the collapse of gas clouds and ignite bursts of new star formation. Blue ribbons of brilliant, young, massive stars can be seen within these gaseous tails, far from the main galactic disk.
Observatories in the Atacama Desert, including the European Southern Observatory's (ESO) Very Large Telescope (VLT), have been instrumental in studying this phenomenon. The GAs Stripping Phenomena in galaxies with MUSE (GASP) survey uses the VLT's MUSE instrument to create detailed 3D maps of these galaxies. These observations show knots of star formation lighting up tentacles that can be 90,000 light-years long or more. Studies of galaxies like JO206 and JW100 reveal the complex interaction of forces that both removes and compresses gas simultaneously. The existence of starbirth under such extreme conditions provides astronomers with a unique laboratory for studying how stars form.