The thermal imprint of galaxy formation on X-ray clusters

It is widely believed that structure in the Universe evolves hierarchically—fluctuations in the primordial distribution of matter, amplified by gravity, collapse and merge to form progressively larger systems, culminating in the clusters of galaxies that are observed today. But the observed structure and evolution of X-ray-emitting clusters of galaxies seems to be at odds with this picture. In particular, clusters and groups with relatively few galaxies, as well as most distant clusters, are substantially fainter in X-rays than predicted by models of hierarchical formation. Here we show that these discrepancies arise because the entropy of the hot diffuse intracluster gas near the centre of the cluster is higher than can be explained by gravitational collapse alone. We argue that the excess entropy is a relic of the energetic winds generated by supernovae in the forming galaxies. These winds also enriched the intracluster medium with elements heavier than helium. We show that such a process can account for the observed effects only if the intracluster medium is heated at modest redshifts (z ≲ 2) but before the final collapse into a cluster structure, indicating that the formation of galaxies precedes that of clusters and that most clusters have been assembled very recently.