| Summary: | The galaxy cluster SPT-CL J0205–5829 currently has the highest spectroscopically confirmed redshift, z = 1.322, in the South Pole Telescope Sunyaev-Zel'dovich (SPT-SZ) survey. XMM-Newton observations measure a core-excluded temperature of T_X = 8.7^(+1.0)_(–0.8) keV producing a mass estimate that is consistent with the Sunyaev-Zel'dovich-derived mass. The combined SZ and X-ray mass estimate of M_(500) = (4.8 ± 0.8) × 10^(14) h^(–1)_(70) M_☉ makes it the most massive known SZ-selected galaxy cluster at z > 1.2 and the second most massive at z > 1. Using optical and infrared observations, we find that the brightest galaxies in SPT-CL J0205–5829 are already well evolved by the time the universe was <5 Gyr old, with stellar population ages ≳3 Gyr, and low rates of star formation (<0.5 M_☉ yr^(–1)). We find that, despite the high redshift and mass, the existence of SPT-CL J0205–5829 is not surprising given a flat ΛCDM cosmology with Gaussian initial perturbations. The a priori chance of finding a cluster of similar rarity (or rarer) in a survey the size of the 2500 deg^2 SPT-SZ survey is 69%. © 2013 American Astronomical Society. Received 2012 May 29; accepted 2012 November 4; published 2013 January 14. The South Pole Telescope program is supported by the National Science Foundation through grant ANT-0638937. Partial support is also provided by the NSF Physics Frontier Center grant PHY-0114422 to the Kavli Institute of Cosmological Physics at the University of Chicago, the Kavli Foundation, and the Gordon and Betty Moore Foundation. Galaxy cluster research at Harvard is supported by NSF grant AST-1009012. Galaxy cluster research at SAO is supported in part by NSF grants AST-1009649 and MRI-0723073. The McGill group acknowledges funding from the National Sciences and Engineering Research Council of Canada, Canada Research Chairs program, and the Canadian Institute for Advanced Research. X-ray research at the CfA is supported through NASA Contract NAS 8-03060. The Munich group ...
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