We present the results of near-infrared spectroscopy of H \alpha emitters ( HAEs ) associated with two protoclusters around radio galaxies ( PKS1138-262 at z =2.2 and USS1558-003 at z =2.5 ) with Multi-Object Infrared Camera and Spectrograph ( MOIRCS ) on the Subaru telescope .
Among the HAE candidates constructed from our narrow-band imaging , we have confirmed membership of 27 and 36 HAEs for the respective protoclusters , with a success rate of 70 per cent of our observed targets .
The large number of spectroscopically confirmed members per cluster has enabled us for the first time to reveal the detailed kinematical structures of the protoclusters at z > 2 .
The clusters show prominent substructures such as clumps , filaments and velocity gradients , suggesting that they are still in the midst of rapid construction to grow to rich clusters at later times .
We also estimate dynamical masses of the clusters and substructures assuming their local virialization .
The inferred masses ( \sim 10 ^ { 14 } M _ { \odot } ) of the protocluster cores are consistent with being typical progenitors of the present-day most massive class of galaxy clusters ( \sim 10 ^ { 15 } M _ { \odot } ) if we take into account the typical mass growth history of clusters .
We then calculated the integrated star formation rates of the protocluster cores normalized by the dynamical masses , and compare these with lower redshift descendants .
We see a marked increase of star-forming activities in the cluster cores , by almost three orders of magnitude , as we go back in time to 11 billion years ago ; this scales as ( 1 + z ) ^ { 6 } .