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

Abstract

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 ≈10^-14ergcm^-2s^-1 near z≈ 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 6σ 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^-15ergcm^-2s^-1. The survey covers 41.2 deg² of overlapping Chandra and XMM-Newton fields and 122.2 deg² 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}/{z_0}}. 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.