Magnetic dynamos in galaxy clusters: The crucial role of galaxy formation physics at high redshifts

¹ Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Golm, Germany
³ Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen, Denmark
⁴ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85740 Garching, Germany

^⋆ Corresponding author.

Received: 30 October 2024
Accepted: 4 July 2025

Abstract

Observations of Faraday rotation and synchrotron emission in galaxy clusters imply large-scale magnetic fields with μG strengths possibly extending back to z = 4. Non-radiative cosmological simulations of galaxy clusters show a comparably slow magnetic field growth that only saturates at late times. We investigated the effects of including galaxy formation physics and found a significantly accelerated magnetic field growth. After adiabatically compressing the magnetic seed fields, we observed further amplification by a fluctuation dynamo until reaching approximate energy equipartition with the turbulent flow. We identified three crucial stages in the magnetic field evolution. 1) At high redshift, the central dominant galaxy serves as the prime agent that magnetizes not only its immediate vicinity but also most of the forming protocluster through a combination of a small-scale dynamo induced by gravitationally driven compressive turbulence and stellar and active galactic nuclei feedback that distributes the magnetic field via outflows. 2) This process continues as other galaxies merge into the forming cluster in subsequent epochs, thereby transporting their previously amplified magnetic field to the intracluster medium through ram pressure stripping and galactic winds. 3) At lower redshift, gas accretion and frequent cluster mergers trigger additional small-scale dynamo processes, thereby preventing the decay of the magnetic field and fostering the increase of the magnetic coherence scale. We show that the magnetic field observed today in the weakly collisional intracluster medium (ICM) is consistently amplified on collisional scales. Initially, this occurs in the collisional interstellar medium during protocluster assembly and later in the ICM on the magnetic coherence scale, which always exceeds the particle mean free path and thus supports the use of magneto-hydrodynamics for studying the cluster dynamo. We generated synthetic Faraday rotation measure observations of protoclusters, and thereby we highlight the potential for studying magnetic field growth during the onset of cluster formation at cosmic dawn.