The Arcminute Microkelvin Imager

The Arcminute Microkelvin Imager is a dual array interferometer designed specifically to survey for galaxy clusters through detecting their Sunyaev-Zel’dovich signatures on the microwave background. Construction of the Small Array is now complete and commissioning observations have mapped several clusters. The Large Array is required to remove the effects of confusing radio sources and will allow deep, blind surveys for clusters. 1. Motivation for a Galaxy Cluster Survey Clusters of galaxies are the most massive collapsed objects in the Universe. They are sensitive probes of structure formation, both in the linear regime of growth and when merging, shocking and gradual virialisation occur. Determining the evolution of clusters, their mass function and structures is therefore of basic importance. A population of clusters at z > 1 is now known to exist (Rosati et al. 2002). However, more data and in particular an unbiased cluster survey are needed to understand cluster formation and early evolution at these redshifts. Optical and X-ray observations suffer from confusion and from a bias towards concentration of mass; another approach is to survey for clusters through detecting their Sunyaev–Zel’dovich (S–Z) effect and several dedicated SZ survey instruments are now being built (see e.g. Mohr et al 2000; Lo 2002; Mohr et al. 2003; Kosowsky 2003; Kneissl et al. 2001). 2. The Sunyaev-Zel’dovich Effect The S–Z effect (Sunyaev & Zel’dovich 1972) is a secondary anisotropy on the Cosmic Microwave Background (CMB) radiation due to inverse-Compton scattering of CMB photons by hot plasma in the gravitational potential of a cluster of galaxies. The S–Z effect has been detected by a number of different groups using a variety of observing techniques (e.g. Birkinshaw & Hughes 1994; Myers et al. 1997; Reese et al. 2000; Komatsu et al. 1999; Pointecouteau et al. 1999; Holzapfel et al. 1997; Grainge et al. 1993; Lancaster et al. 2005; Udomprasert et al. 2004. See e.g. Birkinshaw 1999 or Carlstrom et al. 2002 for a full review). CMB photons are up-scattered in energy with the result that, at frequencies below 220 GHz, one sees a decrease in the CMB temperature towards the cluster. In the Rayleigh-Jeans regime this dip is given by