The thermal SZ effect signal from a cluster is given by
Two types of SZ effect observation of the XMM LSS survey field will be made. We will map the LX > 1038 W clusters, initially using the OCRA-p receiver currently being developed for the Torun 32-m telescope. This instrument, and its upgrade, OCRA-f, which will be available 1 year later, will be able to detect SZ effects of 100 K on arcminute scales in a few hours of observing. Although confusion by the radio source environment of the cluster will limit the accuracy of the OCRA measurements on their own, we will also make VLA high-frequency maps of the clusters, and these will allow good radio source corrections. Additional SZ effect observations, with AMiBA, will be made during AMiBA's commissioning phase, and observations with other SZ-capable instruments (such as SCUBA and CARMA) will be sought for clusters of particular interest. Such observations should provide 10-20 arcsec scale maps which would be interpreted on an individual basis in comparison with the X-ray images of the clusters. These observations will measure cluster SZ effects which can be combined with the integrated XMM fluxes and models of cluster atmospheres (with some parameters fitted to the XMM structures) to produce a Hubble diagram with 23 points at redshifts to greater than 1. This should be sufficient to provide a measurement of the Hubble constant to about 5 per cent accuracy, and would easily show the curvature implied by the recent supernova results which suggest an accelerating Universe. Detailed structural studies of these clusters in the X-ray and SZ effects, combined with optical images (to provide weak shear maps), will measure how the baryonic mass fraction in clusters varies as a function of z, a measurement that is of fundamental importance for assessing the manner in which clusters are formed.
On a longer timescale, the entire LSS field will be surveyed by OCRA, providing a blind SZ effect survey with a resolution of about 1 arcmin and a sensitivity of 100 K, and by the Planck satellite, which will provide a blind SZ effect survey with a resolution of 5 arcmin and a sensitivity of 25 K. While with Planck the beam dilution of the highest-redshift clusters in the XMM survey is severe, OCRA's higher angular resolution is well matched to the angular sizes of clusters to z > 2. With the combined surveys we should easily detect clusters with central SZ effects > 200 K at z > 1. Objects not detected by XMM but appearing in the microwave background surveys would be cool, baryon-rich, clusters (or unusually extended, low-surface brightness clusters): a combined blind X-ray/microwave background survey is the best way to discover such objects if they exist. For low-redshift clusters, Planck's beam dilution is less severe but OCRA's beam-switching leads to reduced efficiency, we will detect all clusters with X-ray luminosities above about 3 1036 W and z < 0.5. This will allow us to build up a baryonic audit of clusters of galaxies, to be compared with the nucleosynthesis results, and hence to provide further clues about the location of unseen matter in the Universe.
The time-scales of the observations:
|OCRA-p||start mid-2002, high-sensitivity pointed observations winter 2002|
|OCRA-f||(improved version of OCRA-p), start high-sensitivity mapping in winter 2004 (at latest)|
|Planck||2008 first data available|
|AMiBA||Expect trial mapping with full system in 2004. Good SZ mapping should follow quickly.|
|SCUBA||Observations are possible whenever we have a source list that can be relied on. We just have to propose for the time.|