A blind detection of a large, complex, Sunyaev–Zel’dovich structure
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Copyright © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society
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 µJy beam-1. In follow-up deep pointed observations the SZ effect is detected with a maximum peak decrement greater than eight 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 (˜10 arcmin) 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 ß model, which does not do justice to this decrement. Fitting an ellipsoid to the deeper candidate we find the following. (a) Assuming that the Evrard et al. approximation to Press & Schechter 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 × 104:1; alternatively assuming Jenkins et al. as the true prior, the formal Bayesian probability ratio of detection is 2.1 × 105:1. (b) The cluster mass is MT,200 = 5.5+1.2 -1.3 × 1014h -1 70 M. (c) Abandoning a physical model with number density prior and instead simply modelling the SZ decrement using a phenomenological ß model of temperature decrement as a function of angular distance, we find a central SZ temperature decrement of -295+36 -15 µK – this allows for cosmic microwave background primary anisotropies, receiver noise and radio sources.We are unsure if the cluster system we observe is a merging system or two separate clusters.
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