A SINFONI view of flies in the Spiderweb: a galaxy cluster in the making

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 Spectrograph for INtegral Field Observations in the Near Infrared (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 δ g ∼ 200 and a matter overdensity of δ m ∼ 70; the latter is similar to that of the outskirts of local galaxy clusters. The velocity distribution of the confirmed satellite galaxies shows a broad, double-peaked velocity structure with σ = 1360 ± 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 ± 87 km s ―1 is found. We show that the protocluster has likely decoupled from the Hubble flow and is a dynamically evolved structure. A comparison to the Millennium 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 the merger of two subclusters can result in an increase in star formation and active galactic nucleus activity in the protocluster core, therefore possibly being an important stage in the evolution of massive cD galaxies.