A first determination of the surface density of galaxy clusters at very low x-ray fluxes

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Abstract
We present the first results of a serendipitous search for clusters of galaxies in deep ROSAT-PSPC pointed observations at high galactic latitude. The survey is being carried out using a Wavelet based Detection Algorithm which is not biased against extended, low surface brightness sources. A new flux--diameter limited sample of 10 cluster candidates has been created from msim 3 , deg^2 surveyed area. Preliminary CCD observations have revealed that a large fraction of these candidates correspond to a visible enhancement in the galaxy surface density, and several others have been identified from other surveys. We believe these sources to be either low--moderate redshift groups or intermediate to high redshift clusters. We show X-ray and optical images of some of the clusters identified to date. We present, for the first time, the derived number density of the galaxy clusters to a flux limit of m 1cdot 10^{-14} erg cm^{-2} s^{-1} (0.5--2.0 keV). This extends the log N--log S of previous cluster surveys by more than one decade in flux. Results are compared to theoretical predictions for cluster number counts
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