Planck early results. IX. XMM-Newton follow-up for validation of Planck cluster candidates

N. Aghanim, M. Arnaud, M. Ashdown, J. Aumont, C. Baccigalupi, A. Balbi, A. J. Banday, R. B. Barreiro, M. Bartelmann, J.G. Bartlett, E. Battaner, K. Benabed, A. Benoît, J.-P. Bernard, M. Bersanelli, R. Bhatia, J. J. Bock, A. Bonaldi, J. R. Bond, J. Borrill, F. R. Bouchet, M. L. Brown, M. Bucher, C. Burigana, P. Cabella, C. M. Cantalupo, J.-F. Cardoso, P. Carvalho, A. Catalano, L. Cayón, A. Challinor, A. Chamballu, L.-Y. Chiang, G. Chon, P. R. Christensen, E. Churazov, D. L. Clements, S. Colafrancesco, S. Colombi, F. Couchot, A. Coulais, B. P. Crill, F. Cuttaia, A. C. da Silva, H. Dahle, L. Danese, P. de Bernardis, G. de Gasperis, A. de Rosa, G. de Zotti, J. Delabrouille, J.-M. Delouis, F.-X. Désert, J. M. Diego, K. Dolag, S. Donzelli, O. Doré, U. Dörl, M. Douspis, X. Dupac, G. Efstathiou, T. A. Enβlin, F. Finelli, I. Flores-Cacho, O. Forni, M. Frailis, E. Franceschi, S. Fromenteau, S. Galeotta, K. Ganga, R. T. Génova-Santos, M. Giard, G. Giardino, Y. Giraud-Héraud, J. González-Nuevo, R. González-Riestra, K. M. Gorski, S. Gratton, A. Gregorio, A. Gruppuso, D. L. Harrison, P. Heinämäki, S. Henrot-Versillé, C. Hernández-Monteagudo, D. Herranz, S. R. Hildebrandt, E. Hivon, M. Hobson, W. A. Holmes, W. Hovest, R. Hoyland, K. M. Huffenberger, G. Hurier, A. H. Jaffe, M. Juvela, E. Keihänen, R. Keskitalo, T. S. Kisner, R. Kneissl, L. Knox, H. Kurki-Suonio, G. Lagache, J.-M. Lamarre, A. Lasenby, R. J. Laureijs, C. R. Lawrence, M. Le Jeune, S. Leach, R. Leonardi, A. R. Liddle, M. Linden-Vørnle, M. López-Caniego, P. M. Lubin, J. F. Macías-Pérez, B. Maffei, D. Maino, N. Mandolesi, R. G. Mann, M. Maris, F. Marleau, E. Martínez-González, S. Masi, S. Matarrese, F. Matthai, P. Mazzotta, A. Melchiorri, J.-B. Melin, L. Mendes, A. Mennella, S. Mitra, M.-A. Miville-Deschênes, A. Moneti, L. Montier, G. Morgante, D. Mortlock, D. Munshi, J. A. Murphy, P. Naselsky, P. Natoli, C. B. Netterfield, H. U. Nørgaard-Nielsen, F. Noviello, D. Novikov, I. Novikov, S. Osborne, F. Pajot, F. Pasian, G. Patanchon, O. Perdereau, L. Perotto, F. Perrotta, F. Piacentini, M. Piat, E. Pierpaoli, R. Piffaretti, S. Plaszczynski, E. Pointecouteau, G. Polenta, N. Ponthieu, T. Poutanen, G. W. Pratt, G. Prézeau, S. Prunet, J.-L. Puget, R. Rebolo López, M. Reinecke, C. Renault, S. Ricciardi, T. Riller, I. Ristorcelli, G. Rocha, C. Rosset, J. A. Rubiño-Martin, B. Rusholme, E. Saar, M. Sandri, D. Santos, B. M. Schäfer, D. Scott, M. D. Seiffert, G. F. Smoot, J.-L. Starck, F. Stivoli, V. Stolyarov, R. Sunyaev, J.-F. Sygnet, J. A. Tauber, L. Terenzi, L. Toffolatti, M. Tomasi, J.-P. Torre, M. Tristram, J. Tuovinen, L. Valenziano, L. Vibert, P. Vielva, F. Villa, N. Vittorio, B. D. Wandelt, S. D. M. White, D. Yvon, A. Zacchei, A. Zonca

Abstract
We present the XMM-Newton follow-up for confirmation of Planck cluster candidates. Twenty-five candidates have been observed to date using snapshot (~10 ks) exposures, ten as part of a pilot programme to sample a low range of signal-to-noise ratios (4 < S/N < 6), and a further 15 in a programme to observe a sample of S/N > 5 candidates. The sensitivity and spatial resolution of XMM-Newton allows unambiguous discrimination between clusters and false candidates. The 4 false candidates have S/N = 4.1. A total of 21 candidates are confirmed as extended X-ray sources. Seventeen are single clusters, the majority of which are found to have highly irregular and disturbed morphologies (about ~70%). The remaining four sources are multiple systems, including the unexpected discovery of a supercluster at z = 0.45. For 20 sources we are able to derive a redshift estimate from the X-ray Fe K line (albeit of variable quality). The new clusters span the redshift range 0.09 ≤ z ≤ 0.54, with a median redshift of z ~ 0.37. A first determination is made of their X-ray properties including the characteristic size, which is used to improve the estimate of the SZ Compton parameter, Y₅₀₀. The follow-up validation programme has helped to optimise the Planck candidate selection process. It has also provided a preview of the X-ray properties of these newly-discovered clusters, allowing comparison with their SZ properties, and to the X-ray and SZ properties of known clusters observed in the Planck survey. Our results suggest that Planck may have started to reveal a non-negligible population of massive dynamically perturbed objects that is under-represented in X-ray surveys. However, despite their particular properties, these new clusters appear to follow the Y₅₀₀–Y_X relation established for X-ray selected objects, where Y_X is the product of the gas mass and temperature.

Keywords
cosmology: observations – galaxies: clusters: general – galaxies: clusters: intracluster medium – cosmic background radiation – X-rays: galaxies: clusters

Notes
Planck is a project of the European Space Agency (ESA) with instruments provided by two scientific consortia funded by ESA member states (in particular the lead countries: France and Italy) with contributions from NASA (USA), and telescope reflectors provided in a collaboration between ESA and a scientific consortium led and funded by Denmark.

Astronomy and Astrophysics
Volume 536, Page A9_1
December 2011

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