  The environment of the high-z radio galaxy PKS 1138-262 at z~2.2 is a prime
example of a forming galaxy cluster. We use deep SINFONI data to perform a
detailed study of the kinematics of the galaxies within 60 kpc of the radio
core and we link this to the kinematics of the protocluster on the megaparsec
scale. Identification of optical emission lines shows that 11 galaxies are at
the redshift of the protocluster. The density of line emitters is more than an
order of magnitude higher in the core of the protocluster than the larger scale
environment. This implies a matter overdensity in the core of delta_m~70 which
is similar to the outskirts of local galaxy clusters. The velocity distribution
of the confirmed satellite galaxies shows a broad, double-peaked velocity
structure with sigma=1360+/-206 km/s. A similar broad, double-peaked
distribution was found in a previous study targeting the large scale
protocluster structure, indicating that a common process is acting on both
small and large scales. Including all spectroscopically confirmed protocluster
galaxies, a velocity dispersion of 1013+/-87 km/s is found. We show that the
protocluster has likely decoupled from the Hubble flow and is a dynamically
evolved structure. Comparison to the Millenium simulation indicates that the
protocluster velocity distribution is consistent with that of the most massive
haloes at z~2, but we rule out that the protocluster is a fully virialized
structure based on dynamical arguments and its X-ray luminosity. Comparison to
merging haloes in the Millennium simulation shows that the structure as
observed in and around the Spiderweb galaxy is best interpreted as being the
result of a merger between two massive haloes. We propose that this merger can
result in an increase in star formation and AGN activity in the protocluster
core and is possibly an important stage in the evolution of massive cD
galaxies.

Cosmology and Nongalactic Astrophysics