Program: GN-2006B-Q-44

We have used the unique multiwavelength data available in the 1 square degree Subaru/XMM-Newton Deep Field (the site of the UKIDSS Ultra Deep Survey) to identify 8 clusters and rich groups of galaxies at z~1. We propose to use GMOS to take spectra of candidate members of these clusters to determine velocity dispersions and hence calculate virial masses. This work differs from many other studies of distant clusters because the combination of unparalleled optical/infrared imaging and deep X-ray data, coupled with sophisticated detection algorithms, make us sensitive to structures with relatively low masses, and to those which have not yet developed a well-defined red sequence. This gives us the opportunity to look further down the cluster mass function than can shallower, wide-area surveys and we will therefore be able to probe beyond the most extreme clusters to more typical examples, an understanding of which is essential to a global picture of cluster formation and evolution, and the evolution of galaxies within clusters.