Galaxies can be considered as islands in space, but they are not entirely isolated. Where galaxies assemble to large associations with up to several thousand members, known as galaxy clusters, they are embedded in diffuse regions called the intracluster medium (ICM). In general this medium is of such a low density that it is rarely optically visible. On the other hand the medium is permeated by magnetic fields, which force cosmic-ray electrons which enter the cluster, to propagate on curved trajectories. This movement leads to loss of energy which is emitted in form of synchrotron radiation. This radiation is emitted at long wavelengths in the centimeter to meter range. It forms a 'radio halo' centered on the galaxy cluster and can be detected by sensitive radio telescopes on Earth.
Near-infrared image of the Perseus cluster with the 144 MHz LOFAR radio halo image is overlaid in red. The infrared image was observed by the Euclid satellite. (Source: van Weeren et al. 2024).
With LOFAR a group of radio astronomers led by Reinout van Weeren from Leiden Observatory studied a prominent example of such a 'radio halo'. It was detected towards one of the closest galaxy clusters located in the Perseus constellation (see Figure). The detailed analysis of the halo at different spatial resolutions allowed to find many details, which can be attributed to a past off-center collision with another galaxy cluster and to the movement of the galaxies within the cluster. Its giant size indicate that even in the outskirts of galaxy clusters such as in Perseus enough turbulent energy is available to re-accelerate electrons.
Reference: Van Weeren et al. 2024, Astronomy & Astrophysics 692, A12
