We report the discovery of a remarkable concentration of massive galaxies with extended X-ray emission at z _ { spec } = 2.506 , which contains 11 massive ( M _ { * } \gtrsim 10 ^ { 11 } M _ { \odot } ) galaxies in the central 80kpc region ( 11.6 \sigma overdensity ) .
We have spectroscopically confirmed 17 member galaxies with 11 from CO and the remaining ones from H \alpha .
The X-ray luminosity , stellar mass content and velocity dispersion all point to a collapsed , cluster-sized dark matter halo with mass M _ { 200 c } = 10 ^ { 13.9 \pm 0.2 } M _ { \odot } , 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 80kpc core reaches \sim 3400 M _ { \odot } yr ^ { -1 } with a gas depletion time of \sim 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 ( \sim 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 .