* Astronomy

Title: 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
Authors: Planck Collaboration: N. Aghanim, M. Arnaud, F. Atrio-Barandela, J. Aumont, C. Baccigalupi, A. Balbi, A. J. Banday, J. G. Bartlett, E. Battaner, K. Benabed, J.-P. Bernard, M. Bersanelli, R. Bhatia, H. Böhringer, A. Bonaldi, J. R. Bond, S. Borgani, J. Borrill, F. R. Bouchet, M. L. Brown, C. Burigana, P. Cabella, P. Carvalho, L.-Y Chiang, C. Chiang, G. Chon, E. Churazov, S. Colafrancesco, S. Colombi, B. P. Crill, F. Cuttaia, A. Da Silva, H. Dahle, L. Danese, O. D'Arcangelo, R. J. Davis, P. de Bernardis, G. de Gasperis, G. de Zotti, J. Delabrouille, J.-M. Delouis, J. Démoclès, F.-X. Désert, C. Dickinson, J. M. Diego, H. Dole, S. Donzelli, O. Doré, M. Douspis, X. Dupac, G. Efstathiou, T. A. En\sslin, H. K. Eriksen, F. Finelli, I. Flores-Cacho, O. Forni, P. Fosalba, et al. (94 additional authors not shown)

We present first results on PLCK G266.6-27.3, 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 is a 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_500 = (7.8 ±0.8)e+14 solar masses. PLCK G266.6-27.3 is an exceptional system: its luminosity of L_X(0.5-2.0 keV)=(1.4 ±0.05)e+45 erg/s, 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, PLCK G266.6-27.3 appears to be highly relaxed. This observation confirms Planck's capability to detect high redshift, high mass clusters, and opens the way to the systematic study of population evolution in the exponential tail of the mass function.

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