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We present new spectroscopic observations of the most distant X-ray selectedgalaxy cluster currently known, XMMXCS J2215.9-1738 at z=1.457, obtained withthe DEIMOS instrument at the W. M. Keck Observatory, and the FORS2 instrumenton the ESO Very Large Telescope. Within the cluster virial radius, as estimatedfrom the cluster X-ray properties, we increase the number of knownspectroscopic cluster members to 17 objects, and calculate the line of sightvelocity dispersion of the cluster to be 580+/-140 km/s. We find mild evidencethat the velocity distribution of galaxies within the virial radius deviatesfrom a single Gaussian. We show that the properties of J2215.9-1738 areinconsistent with self-similar evolution of local X-ray scaling relations,finding that the cluster is underluminous given its X-ray temperature, and thatthe intracluster medium contains ~2-3 times the kinetic energy per unit mass ofthe cluster galaxies. These results can perhaps be explained if the cluster isobserved in the aftermath of an off-axis merger. Alternatively, heating of theintracluster medium through supernovae and/or Active Galactic Nuclei activity,as is required to explain the observed slope of the local X-rayluminosity-temperature relation, may be responsible.

The XMM Cluster Survey: The Dynamical State of XMMXCS J2215.9-1738 at z=1.457