The supersonic nature of jellyfish galaxies

All gas-rich galaxies in cluster environments are expected to experience ram-pressure stripping from the intracluster 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 extraplanar star formation, and we explore 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 discovered 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, suggesting a possible association between magnetic field draping, shock compression of the gas, and extraplanar star formation activity. We then tested this scenario in a sample of 17 jellyfish galaxies from the GASP project. We combined dynamical models for their orbits within the host clusters with realistic cluster temperature profiles to infer their Mach number, and we found 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 evaporation of the stripped gas 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.