Discovery of the most distant X-ray galaxy cluster in the universe, and its implications on galaxy formation and cosmology

Galaxy clusters trace the densest galaxy environment and the most massive dark matter halos, hence are ideal laboratories to study environmental effects on galaxy evolution. Local galaxy clusters are exclusively dominated by massive ellipticals in the cluster core. Both galaxy cluster archaeology and cosmological simulations suggest that these galaxies formed in a short time scale at z>~2, however, clear evidence for structures in such a rapid formation phase has been lacking so far, and the (trans)formation mechanisms of cluster ellipticals remain a mystery. In this talk I will present our recent discovery of the most distant galaxy cluster known to date at z=2.51(named “CLJ1001”). In addition to its record-breaking distance, CLJ1001 is the first X-ray cluster discovered so far that is dominated by massive star-forming galaxies in the core (instead of massive ellipticals found in low-redshift clusters). With an exceptional star formation rate of ~3400 Msun/yr and a gas depletion time of ~200 Myrs, CLJ1001 was caught in a rapid formation phase of central cluster ellipticals, and likely represents a critical transition phase between young, proto-clusters and mature clusters. I will discuss the implications of CLJ1001 on the formation mechanisms of cluster ellipticals. I will also show that, with the prevalence of both starbursts and (radio-) AGN activities, the discovery of CLJ1001 initiates a novel science case for SKA, which will help to revolutionize studies of high-z (proto-)clusters in the next decade.