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K Jahoda, R Petre, R. F. Mushotzky, K. Gendreau, N. E. White (NASA/GSFC)
We present a MIDEX mission concept, on behalf of an international collaboartion, with the capability of answering numerous fundamental questions about the dark matter which dominates the gravitational mass of the universe, including: How much dark matter is there? What is its spatial distribution? Is the large scale structure that is observed today the result of gravitational instabilities (as assumed by all current theories)? The basis of our concept is an X-ray survey of \pi sr with the following characteristics: 0.5-10 keV bandpass, 10 arc-sec angular resolution, and (50 photon) flux limit 5 \times 10-14 erg/s/cm2 (0.5-2 keV band). The X-ray data will be of sufficient quality to separate the primary dark matter tracers - clusters of galaxies - from the background population of AGN; the mission observation strategy will minimize the time required to measure the third dimension, redshift, by concentrating on the Sloan Digital Sky Survey region, thus performing a large fraction of the optical "follow-up" in advance. Based on the observed log N-log S distribution for clusters, we expect to detect substantially more than 104 clusters of galaxies.