1012.4441 (AMI Consortium: T. W. Shimwell et al.)

A blind detection of a large, complex, Sunyaev--Zel'dovich structure    [PDF]

AMI Consortium: T. W. Shimwell, R. W. Barker, P. Biddulph, D. Bly, R. C. Boysen, A. R. Brown, M. L. Brown, C. Clementson, M. Crofts, T. L. Culverhouse, J. Czeres, R. J. Dace, M. L. Davies, R. D'Alessandro, P. Doherty, K. Duggan, J. A. Ely, M. Felvus, F. Feroz, W. Flynn, T. M. O. Franzen, J. Geisbusch, R. Genova-Santos, K. J. B. Grainge, W. F. Grainger, D. Hammett, M. P. Hobson, C. M. Holler, N. Hurley-Walker, R. Jilley, T. Kaneko, R. Kneissl, K. Lancaster, A. N. Lasenby, P. J. Marshall, F. Newton, O. Norris, I. Northrop, D. M. Odell, M. Olamaie, Y. C. Perrott J. C. Pober, G. G. Pooley, M. W. Pospieszalski, V. Quy, C. Rodriguez-Gonzalvez, R. D. E. Saunders, A. M. M. Scaife, M. P. Schammel, J. Schofield, P. F. Scott, C. Shaw, H. Smith, D. J. Titterington, M. Velic, E. M. Waldram, S. West, B. A. Wood, G. Yassin, J. T. L. Zwart

We present an interesting Sunyaev-Zel'dovich (SZ) detection in the first of the Arcminute Microkelvin Imager (AMI) 'blind', degree-square fields to have been observed down to our target sensitivity of 100{\mu}Jy/beam. In follow-up deep pointed observations the SZ effect is detected with a maximum peak decrement greater than 8 \times the thermal noise. No corresponding emission is visible in the ROSAT all-sky X-ray survey and no cluster is evident in the Palomar all-sky optical survey. Compared with existing SZ images of distant clusters, the extent is large (\approx 10') and complex; our analysis favours a model containing two clusters rather than a single cluster. Our Bayesian analysis is currently limited to modelling each cluster with an ellipsoidal or spherical beta-model, which do not do justice to this decrement. Fitting an ellipsoid to the deeper candidate we find the following. (a) Assuming that the Evrard et al. (2002) approximation to Press & Schechter (1974) correctly gives the number density of clusters as a function of mass and redshift, then, in the search area, the formal Bayesian probability ratio of the AMI detection of this cluster is 7.9 \times 10^4:1; alternatively assuming Jenkins et al. (2001) as the true prior, the formal Bayesian probability ratio of detection is 2.1 \times 10^5:1. (b) The cluster mass is MT,200 = 5.5+1.2\times 10^14h-1M\odot. (c) Abandoning a physical model with num- -1.3 70 ber density prior and instead simply modelling the SZ decrement using a phenomenological {\beta}-model of temperature decrement as a function of angular distance, we find a central SZ temperature decrement of -295+36 {\mu}K - this allows for CMB primary anisotropies, receiver -15 noise and radio sources. We are unsure if the cluster system we observe is a merging system or two separate clusters.

View original: http://arxiv.org/abs/1012.4441