Clusters of galaxies are massive cosmic structures containing 100s to 1000s of galaxies. Most of the visible material in a cluster is in the form of a hot, X-ray emitting plasma, but about 90% of the mass of a cluster is dark matter. We use observations of the different components of galaxy clusters to study their formation and evolution and the complex feedback mechanisms between the X-ray plasma, the galaxies, and their active galactic nuclei. We are also working on refining techniques for determining the masses of galaxy clusters, including their dark matter.

Much of this work is done as part of major international collaborations performing surveys to discover new, distant, clusters of galaxies (such as XXL and surveys with Euclid), or studying known clusters of galaxies in exquisite detail (such as the CHEX-MATE collaboration).

Fig.1 is an X-ray image showing the early stages of a merger between two massive subclusters (the two large, bright components in the centre) at a redshift of z=0.83. The cluster is part of a larger scale structure comprising at least two galaxy groups (fainter components towards the top right and bottom left). Many of the smaller sources in this image are active galactic nuclei in galaxies in the cluster, or in the foreground or background.

Galaxy cluster masses

Clusters of galaxies can be used as powerful cosmological probes with which to measure the fundamental properties of our Universe. To do this effectively, their masses must be measured. This is an observational challenge as clusters are dominated by dark matter, and we must estimate their masses from our observations of their luminous matter. By combining observations of the hot gas in clusters made with X-ray observatories, with measurements of the Sunyaev-Zel'dovich effect, measurements of the gravitational lensing effect of clusters, and the velocities of the galaxies within clusters, we are attempting to measure the most accurate masses possible for large numbers of galaxy clusters.

Advanced statistical modelling

We use advanced statistical techniques to model large populations of galaxy clusters to investigate the way that astrophysical processes have altered the scaling relationships between their properties. Detailed analysis is required to model the biases that are introduced by the selection of clusters to study. We also work on developing new statistical techniques to detect galaxy clusters in large, multi-wavelength survey datasets.