EVIDENCE FOR NONLINEAR GROWTH OF STRUCTURE FROM AN X-RAY-SELECTED CLUSTER SURVEY USING A NOVEL JOINT ANALYSIS OF THE CHANDRA AND XMM-NEWTON ARCHIVES

We present a large X-ray-selected serendipitous cluster survey based on a novel joint analysis of archival Chandra and XMM-Newton data. The survey provides enough depth to reach clusters of flux of approximate to 10(-14) erg cm(-2) s(-1) near z approximate to 1 and simultaneously a large-enough sample to find evidence for the strong evolution of clusters expected from structure formation theory. We detected a total of 723 clusters of which 462 are newly discovered clusters with greater than 6 sigma significance. In addition, we also detect and measure 261 previously known clusters and groups that can be used to calibrate the survey. The survey exploits a technique that combines the exquisite Chandra imaging quality with the high throughput of the XMM-Newton telescopes using overlapping survey regions. A large fraction of the contamination from active galactic nucleus point sources is mitigated by using this technique. This results in a higher sensitivity for finding clusters of galaxies with relatively few photons and a large part of our survey has a flux sensitivity between 10(-14) and 10(-15) erg cm(-2) s(-1). The survey covers 41.2 deg(2) of overlapping Chandra and XMM-Newton fields and 122.2 deg(2) of non-overlapping Chandra data. We measure the log N-log S distribution and fit it with a redshift-dependent model characterized by a luminosity distribution proportional to e(-z/z0). We find z(0) to be in the range 0.7-1.3, indicative of rapid cluster evolution, as expected for cosmic structure formation using parameters appropriate to the concordance cosmological model. Confirmation of our cluster detection efficiency through optical follow-up studies currently in progress will help to strengthen this conclusion and eventually allow to use these data to derive tight constraints on cosmological parameters.