A FIRST DETERMINATION OF THE SURFACE DENSITY OF GALAXY CLUSTERS AT VERY LOW X--RAY FLUXES

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 $\sim 3\mathrm{de}{\mathrm{g}}^2$ 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 $1\cdot 10^{-14}\mathrm{ergc}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ (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.

uuencoded compressed Postscript, 7 pages including 4 figures. Accepted for publication in Ap. J. Letters.