As the Universe evolves, gravity brings together hundreds, sometimes thousands of galaxies together to form galaxy clusters. The galaxies within these clusters usually account for about 1% of the total mass. They are encompassed by a hot low density gas (a million to 10 million Kelvin) known as the intra-cluster medium (ICM) which contains about 9% of the cluster mass. The other, approximately 90% of the mass is in the surrounding dark matter halo.

Observations at different frequencies

Observations at different frequencies help us to form a comprehensive picture of the structure and evolution of galaxy clusters. Optical telescopes, for example, can detect individual galaxies allowing us to determine the dynamics of the galaxies and infer the distribution of dark matter. X-rays observatories are used to measure thermal emission from the ICM. Radio telescopes offer a completely different view.  They detect non-thermal emission, which reveals the cluster’s magnetic field and the sites of extreme particle acceleration in the ICM.

ASTRON interests

Our group at ASTRON is interested in studying the particle acceleration processes and magnetic fields within the tenuous ICM. We wish to understand the formation of radio halos which are characterised by cluster-wide radio emission and are thought to be caused by turbulence throughout the cluster. We also aim to understand the conditions that lead to the formation of radio relics. These objects are characterised by their peripheral location and are thought to be generated by large shock waves. Finally, we are studying other unusual structures showing intense particle acceleration in the ICM and the interaction between the ICM and discrete radio sources such as tailed radio galaxies.


The images above show the galaxy cluster Abell 2034 in the optical, X-ray and radio. These show that the cluster contains 328 individual galaxies (including two massive brightest cluster galaxies) a disturbed ICM and several distinct sites of particle acceleration.

Research staff

Tim Shimwell

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