Abstract
The evolution of galaxy clusters is most sensitively probed by observational studies of the most distant systems. Clusters and cluster galaxies have undergone relatively gentle evolution at z1 must therefore see their strongest evolution. The best probes of cluster and cluster galaxy evolution require samples identified using a well-understood selection function, one that relies on an observable that relates directly to a key diagnostic of the system. X-ray selection does just this: The clusters are detected though the extended X-ray emission from the hot gas trapped in and tracing the clusters' gravitational wells. The XXL survey is an X-ray survey of 50 sq degrees of sky, designed to indentify galaxy clusters out to z~2 with masses of M>1e14 M_Sun. In this programme we will obtain full 3.6/4.5 micron data to a level deep enough to securly identify all z>1 clusters detected in the northern half of XXL. This IRAC data set crucially enables a variety of detailed evolutionary studies o clusters and their galaxy populations at z>1, including (1) exploring the formation of the red sequence and build-up of cluster galaxies, (2) determining the evolution of the X-ray scaling relations, key probes of the physics driving cluster evolution and (3) determining the number density and X-ray luminosity function of clusters at z>1, a vital step in using the XXL survey to eventually constrain the Dark Energy equation of state at a highly competitive level. The resulting data set, when combined with the wealth of multi-waveband coverage of the same field, will enable many other studies (by us and others) including those of obscured and very distant AGN, CMB lensing studies and even an estimation of the timescale of reionization. As a consequence it has an extremely high legacy value.
| Original language | English |
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| Publication status | Published - 1 Sept 2012 |