Planck early results. XXVI. Detection with Planck and confirmation by XMM-Newton of PLCK G266.6-27.3, an exceptionally X-ray luminous and massive galaxy cluster at z ~ 1

We present first results on PLCKG266.6-27.3, a galaxy cluster candidate detected at a signal-to-noise ratio of 5 in the Planck All Sky survey. An XMM-Newton validation observation has allowed us to confirm that the candidate isa bona fide galaxy cluster. With these X-ray data we measure an accurate redshift, z = 0.94 0.02, and estimate the cluster mass to be M<SUB>500</SUB> = (7.8 0.8) 10<SUP>14</SUP> M<SUB>&sun;</SUB>. PLCKG266.6-27.3 is an exceptional system its luminosity of L<SUB>X</SUB> [0.5-2.0 keV] = (1.4 0.05) 10<SUP>45</SUP> erg s<SUP>-1</SUP> equals that of the two most luminous known clusters in the z > 0.5 universe, and it is one of the most massive clusters at z ~ 1. Moreover, unlike the majority of high-redshift clusters, PLCKG266.6-27.3 appears to be highly relaxed. This observation confirms Planck's capability of detecting high-redshift, high-mass clusters, and opens the way to the systematic study of population evolution in the exponential tail of the mass function. Corresponding author M. Arnaud, monique.arnaud@cea.fr