SummaryThe Survey
Understanding dark energy, the most dominant and least understood component of the universe, is the single greatest goal in cosmology (and arguably, all of physics) for the next decade. Measuring the dark energy equation of state, encapsulated by the w parameter, and its evolution is the goal of a number of on-going and planned experiments. Recent modeling highlights the fact that measuring the abundance of massive galaxy clusters is an essential complementry component of this effort. We are thus carrying out a 1000 sq. deg cluster survey using MegaCam, applying the demonstrably very successful techniques developed for the now-completed Red Sequence Cluster Survey, with the goal of determining w to a precision of 10% (from clusters alone). This survey will provide constraints in the w--ΩM plane which are orthogonal and of similar uncertainty to SNe constraints (e.g., from the CFHT-LS). We have also started a parallel effort to link optical richness (measurable from the survey data) to cluster mass, including dynamics, X-ray, weak lensing, and NIR studies. Weak lensing analysis using both the survey and followup data will be used as our primary mass-richness calibrator. Our second major science goal is the discovery of 50-100 new strong lensing clusters -- a powerful and important new tool for studying the early stages of galaxy formation at high redshifts. Background
The observational evidence for dark energy as a dominant component of the universe has been steadily increasing in the past few years, most notably thanks to improved measurements of the cosmic microwave background and high-redshift SNe (e.g., [14],[20a]). Arguably, the single greatest task facing cosmology in this decade is to improve our understanding of the dark energy by placing constraints on the way it evolves with time. A first step in this direction is the measurement of the equation of state of the dark energy w = P/ρ. Most current efforts, instigated by the type Ia SNe results (e.g., [20]), aim to constrain w by measuring the distance-redshift relation (e.g., the SuperNova Cosmology Project[25], the CFHTLS[26], or proposed surveys to measure baryon oscillations in the galaxy power spectrum [2]). An alternative route is to measure the growth of structure as a function of redshift. It is a fundamentally different approach, which, in conjunction with "distance" methods, can provide a unique test of general relativity on large scales as well. For instance, cosmic shear studies (e.g., the CFHTLS) aim to constrain w this way. Given the importance of a successful determination of w, several independent methods should be used. Of these, the measurement of the abundance of clusters of galaxies as a function of mass and redshift has attracted much attention (e.g., see review in [17]). The cluster mass function is an excellent probe due to its combined sensitivity to the comoving volume, as well as the growth of large scale structure. The rare, very massive clusters at redshifts z>0.5 provide most of the discriminating power, requiring surveys that are an order of magnitude larger than those existing to date. Many large experiments are being planned, of which the South Pole Telescope Sunyaev-Zeldovich survey (see [17]) is the most relevant. Once operational (earliest, 2007), it aims to cover 4000 deg2. However, this type of observations provides only limited information about the clusters and cluster redshifts need to be obtained independently. To this end, a US team is building a custom wide-field camera to image the surveyed area in the optical (the Dark Energy Survey [28]), which would start observations late-2009 at the earliest. A much quicker and cost-efficient survey of similar coverage is to use optical multi-color imaging alone and the very successful cluster red-sequence technique for cluster finding (Fig. 1 and 2; see [8] for the first cluster catalogs, and [8a] for the first cosmological results from the RCS1). Additional Major Science GoalsThe 90 deg2 of the RCS1 contains a half dozen strong lensing clusters[7] at z>0.5, including several examples with multiple bright arcs. A 1000 deg2 survey of similar depth should discover 50-100 such systems (Fig. 4; 15 have been discovered in the first 120 deg2), by far the largest homogeneous sample of strong lensing clusters. These data will enable us to study star formation and stellar populations of young, forming galaxies in great detail thanks to the large magnifications [e.g., 6,7,24]. Finally, the data provide an ideal data set to study cosmic shear and the properties of dark matter halos from weak lensing (e.g., [12,13]) over a significantly larger area than other on-going surveys. References
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