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 \pm 0.8 ) \times 10 ^ { 14 } h _ { 70 } ^ { -1 } M _ { \sun } 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 > \sim 3 Gyr , and low rates of star formation ( < 0.5 M _ { \sun } /yr ) .
We find that , despite the high redshift and mass , the existence of SPT-CL J0205-5829 is not surprising given a flat \Lambda CDM 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 % .