Identification of the progenitors of rich clusters and member galaxies
  in rapid formation at z>2
  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$.
