Formation and Evolution of
Galaxy Clusters Across Cosmic Time

The hydrodynamical interactions between galaxies and their surrounding gas in clusters is a major driver of galaxy evolution, and nowhere is this interaction more dramatically demonstrated than for narrow-angle tail radio sources (NATs); active galaxies whose radio jets are bent back to an acute angle by their motion through the intracluster medium of galaxy clusters. As such, they offer us an interesting diagnostic of these systems’ orbital motion to study their interaction with the gas. We have therefore used the Low-Frequency Array (LOFAR) Two-metre Sky Survey first data release to compile the largest ever sample of 255 NATs matched to clusters, based on both photometric and spectroscopic redshifts. The spectroscopic subset confirms that line-of-sight contamination remains modest out to 7R500, so we use a sample that excludes BCGs but extends out to these radii, and find a large excess of galaxies with their tails pointing away from the cluster centre. The spectroscopic subsample indicates that this excess persists to at least 15R500. At small radii, we also find a small excess of jets bent toward the cluster centre. The large-radii results are very surprising, as the effects are found far beyond the cluster virial radius where we might expect such hydrodynamical phenomena to occur. They indicate that infalling filaments are dense enough to start to experience hydrodynamic deceleration, and perhaps preferentially trigger AGN activity. We seem to be seeing AGN as they fall radially in down such filaments, and emerge after passing pericentre, making NATs a potential indicator not just for the location of clusters, but also for the filaments that connect them.