Tao Wang, David Elbaz, Emanuele Daddi, Alexis Finoguenov, Daizhong Liu, Corentin Schreiber, Sergio Martín, Veronica Strazzullo, Francesco Valentino, Remco Van Der Burg, Anita Zanella, Laure Ciesla, Raphael Gobat, Amandine Le Brun, Maurilio Pannella, Mark Sargent, Xinwen Shu, Qinghua Tan, Nico Cappelluti, Yanxia Li

Research output: Contribution to journalArticlepeer-review

146 Scopus citations


We report the discovery of a remarkable concentration of massive galaxies with extended X-ray emission at zspec = 2.506, which contains 11 massive (M ≳ 1011 M) galaxies in the central 80 kpc region (11.6σ overdensity). We have spectroscopically confirmed 17 member galaxies with 11 from CO and the remaining ones from Hα. The X-ray luminosity, stellar mass content, and velocity dispersion all point to a collapsed, cluster-sized dark matter halo with mass M200c = 1013.9 ± 0.2 M, making it the most distant X-ray-detected cluster known to date. Unlike other clusters discovered so far, this structure is dominated by star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive galaxies classified as quiescent. The star formation rate (SFR) in the 80 kpc core reaches ∼3400 M yr-1 with a gas depletion time of ∼200 Myr, suggesting that we caught this cluster in rapid build-up of a dense core. The high SFR is driven by both a high abundance of SFGs and a higher starburst fraction (∼25%, compared to 3%-5% in the field). The presence of both a collapsed, cluster-sized halo and a predominant population of massive SFGs suggests that this structure could represent an important transition phase between protoclusters and mature clusters. It provides evidence that the main phase of massive galaxy passivization will take place after galaxies accrete onto the cluster, providing new insights into massive cluster formation at early epochs. The large integrated stellar mass at such high redshift challenges our understanding of massive cluster formation.

Original languageEnglish
Article number56
JournalAstrophysical Journal
Issue number1
StatePublished - 1 Sep 2016
Externally publishedYes


  • galaxies: clusters: general
  • galaxies: evolution
  • galaxies: formation
  • galaxies: high-redshift
  • galaxies: starburst
  • large-scale structure of universe


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