2011ApJ...731...86F

Using the South Pole Telescope (SPT), we have discovered the most massive known galaxy cluster at z>1, SPT-CL J2106-5844. In addition to producing a strong Sunyaev-Zel'dovich (SZ) effect signal, this system is a luminous X-ray source and its numerous constituent galaxies display spatial and color clustering, all indicating the presence of a massive galaxy cluster. Very Large Telescope and Magellan spectroscopy of 18 member galaxies shows that the cluster is at z = 1.132^+0.002_–0.003. Chandra observations obtained through a combined HRC-ACIS GTO program reveal an X-ray spectrum with an Fe K line redshifted by z = 1.18±0.03. These redshifts are consistent with the galaxy colors found in optical, near-infrared, and mid-infrared imaging. SPT-CL J2106-5844 displays extreme X-ray properties for a cluster having a core-excluded temperature of T_X= 11.0^+2.6_–1.9 keV and a luminosity (within r₅₀₀) of L_X(0.5-2.0 keV) = (13.9±1.0)x10⁴⁴ erg/s. The combined mass estimate from measurements of the SZ effect and X-ray data is M₂₀₀= (1.27±0.21)x10¹⁵ h^–1₇₀M_☉. The discovery of such a massive gravitationally collapsed system at high redshift provides an interesting laboratory for galaxy formation and evolution, and is a probe of extreme perturbations of the primordial matter density field. We discuss the latter, determining that, under the assumption of ΛCDM cosmology with only Gaussian perturbations, there is only a 7% chance of finding a galaxy cluster similar to SPT-CL J2106-5844 in the 2500 deg² SPT survey region and that only one such galaxy cluster is expected in the entire sky.