Jellyfish Galaxies: Supersonic Winds Ignite Star Formation express
A galaxy plunges into a dense cluster like a runner into a crowd. The oncoming gas, known as the intracluster medium, strips away its own gas, stretching it into a long, glowing tail. Astronomers call these galaxies jellyfish: they have a body and flowing tentacles of stripped material. But only in a few do new stars ignite within those tentacles. Why are some jellyfish tails alive with star formation while others are just gas trails?
Astronomers took a close look at JO147, a jellyfish with weak star formation. The MeerKAT radio telescope revealed that polarized emission, a sign of an ordered magnetic field, exists only in a tenth of the tail. The galaxy's speed relative to the surrounding gas is modest: just 1.3–1.6 times the speed of sound. For comparison, a supersonic jet travels faster than sound in air, but here "sound" refers to oscillations in the cluster's hot gas.
Next, the team tested this idea on 17 jellyfish from the GASP project. For each, they modeled the orbit within the cluster, accounted for gas temperature, and computed the Mach number — the ratio of the galaxy's speed to the speed of sound. It turned out that the faster the galaxy moves, the more actively stars are born in its tail. Supersonic motion is a necessary condition. The shock wave compresses the stripped gas, preventing it from dissipating, while the amplified magnetic field wraps it like a blanket, preserving fuel for new stars.
Original abstract
as written by the authors arXiv:2602.21821
All gas-rich galaxies in cluster environments are expected to experience ram-pressure stripping from the intra-cluster medium. However, only a fraction of these develop ongoing star-formation in their stripped tail, becoming the so-called ``jellyfish'' galaxies. In this work we provide observational evidence that magnetic fields can signal differences in the extraplanar star formation and explore what are the physical conditions that lead to the formation of a jellyfish galaxy. We first focus on JO147, a jellyfish galaxy that features weak star formation activity in its tail. Using MeerKAT radio continuum observations, we discover polarized emission only in a small fraction of its tail, with an average fraction of $$~10\%$$, and a low Mach number $$\mathcal{M}=1.3-1.6$$, which suggests a possible association between magnetic field draping, shock-compression of the gas, and extraplanar star formation activity. Then, we test this scenario in a sample of 17 jellyfish galaxies from the GASP project. We combine dynamical models for their orbits within the host clusters with realistic cluster temperature profiles to infer their Mach number, and we find a positive correlation between it and the star formation activity in their tail. We conclude that supersonic motion is a necessary condition for triggering star formation in the stripped tails of jellyfish galaxies. Our findings provide empirical evidence that the critical factor preventing the stripped gas evaporation is the shock compression induced by the supersonic motion through the cluster. This process likely enhances the magnetic field surrounding the galaxy and the properties of the stripped material.
A jellyfish galaxy's tail can stretch for hundreds of thousands of light-years — several times the diameter of the Milky Way.
In Terry Goodkind's novel "Pillars of Creation," characters travel through regions where stars are born from compressed gas — similar to the processes in jellyfish galaxy tails.