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 TX = 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 M500 = (4.8±0.8)×1014h−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 deg2 SPT-SZ survey is 69%.
