The supersonic nature of jellyfish galaxies

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, (PD), Italy
² INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, (CA), Italy
³ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, 10010 New York, (NY), USA
⁵ Departamento de Física, Universidad Técnica Federico Santa María, Avenida España 1680 Valparaíso, Chile
⁶ Millennium Nucleus for Galaxies (MINGAL), Valparaíso, Chile
⁷ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155, 2200 Copenhagen, Denmark
⁸ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD, Groningen, The Netherlands
⁹ Department of Physics and Astronomy “Augusto Righi”, University of Bologna, Via Gobetti 93/2, 40129 Bologna, (BO), Italy
¹⁰ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, (MI), Italy
¹¹ University Observatory, LMU Faculty of Physics, Scheinerstrassee 1, 81679 München, Germany
¹² INAF – Istituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, (BO), Italy
¹³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio Bologna, Via Piero Gobetti, 93/3, 40129 Bologna, (BO), Italy
¹⁴ Department of Physics, Faculty of Science, University of Zagreb, Bijenička 32, 10 000 Zagreb, Croatia
¹⁵ Astronomical Institute (AIRUB), Ruhr-University Bochum, Faculty of Physics and Astronomy, 44780 Bochum, Germany

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Received: 17 December 2025
Accepted: 25 February 2026

Abstract

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, 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.