The environment of the high- z radio galaxy PKS 1138-262 at z \sim 2.2 is a prime example of a forming galaxy cluster .
We use deep SINFONI integral field spectroscopy 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 with respect to the larger scale environment .
This implies a galaxy overdensity in the core of \delta _ { g } \sim 200 and a matter overdensity of \delta _ { m } \sim 70 , the latter of 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 \pm 206 km s ^ { -1 } .
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 \pm 87 km s ^ { -1 } 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 \sim 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 the merger of two subclusters can result in an increase in star formation and AGN activity in the protocluster core , therefore possibly being an important stage in the evolution of massive cD galaxies .