REXCESS project details

 | Home | Project details | Table of clusters | Publications | Cluster pages | Team members |
The primary goal of REXCESS is the calibration of the scaling relations for a statistical sample of clusters, selected by X-ray luminosity alone (the criterion most commonly used in cosmological applications of clusters). We also hope to establish the present data as a benchmark sample for studies in other wavelengths. In this context, contrary to most previous studies where researchers would choose the most regular clusters for an intercomparison, we want to provide a special incentive to also observe and reconstruct the more complex, apparently unrelaxed objects with different techniques of structure and mass measurements. Therefore, some of the major goals of this project are to better characterize and understand:
  1. The relations of observables such as X-ray luminosity, temperature, and characteristic radius with cluster mass,
  2. The source of the scatter in these relations,
  3. The dynamical states of the clusters via inspection of temperature, entropy and pressure maps as well as by the comparison of X-ray and optical spectroscopic observations (guided by simulations),
  4. The statistics of cluster mergers and the frequency of cluster cooling cores as a function of cluster mass; both cosmologically very important diagnostics (e.g. Schuecker et al. 2001b),
  5. Entropy profiles of the clusters as probes of the thermal and star formation history in the clusters,
  6. Metal abundances in clusters as a function of various observational parameters, and
  7. The variation of the cluster mass and mass-to-light ratio profiles.

For the construction of an unbiased, X-ray selected cluster sample we use as a parent sample the REFLEX survey catalogue, which is presently the largest, well controlled cluster catalogue (Boehringer et al. 2004). The quality of the sample has been demonstrated by showing that it can provide reliable measures of the large-scale structure without distorting artifacts (Collins et al. 2000, Schuecker et al. 2001a, Kerscher et al. 2003), yielding cosmological parameters in good agreement within the measurement uncertainties with the 3year WMAP results (Schuecker et al. 2003a, b, Stanek et al. 2006, Spergel et al. 2006; note that this good agreement with the new WMAP data is also true for other cluster studies e.g. Voevodkin & Vikhlinin 2004, Henry 2004). Moreover, the study of the galaxy cluster number density and the measured large-scale clustering provide consistent cosmological results. The basic criteria for the selection of the present subsample are the following:

  • We restrict the redshifts to z <0.2 to obtain a census of the local Universe.
  • The basic selection criterion is X-ray luminosity, with no preference for any particular morphological type. Thus the sample should be representative of any local, high quality, unbiased X-ray survey, a survey of the type applicable to cosmological model testing.
  • To best assess the scaling relations, the selection has been designed to provide a close to homogenous coverage of the X-ray luminosity range. The chosen luminosity regime, L_X = 0.407 - 20 x 10^44 h_50^(-2) erg/s in the 0.1-2.4 keV rest frame band, provides clusters with estimated temperatures above 2 keV. Thus the spectrum of selected objects covers the range from poor systems to the most massive clusters. Lower temperature systems, groups of galaxies, are excluded because their study requires a larger observational effort than the handful of additional data points that can be afforded here.
  • We aim for a good global characterization of the clusters, and thus wish to detect cluster emission out to the fiducial radius r_500, the radius inside which the mean cluster mass density is 500 times the critical density of the Universe. This has been shown by simulations to provide one of the best measures of the size of the virialized dark matter system (Evrard et al. 1996).
  • The distances of the objects are selected to optimally use the field-of-view, angular resolution, and photon collection power of the {\sl XMM-Newton} observatory. For the data reduction we use the region of the target fields outside about 10 - 11 arcmin to assess the X-ray background of the observation. This is to enable a comparison of the properties of the target background and the background field to correct for background variations.
  • We use well defined selection criteria such that the space density of the sample and any subset of it is well defined by the selection function.

These selection requirements cannot be met by a simple flux-limit cut. In particular, to meet the condition of a nearly homogeneous luminosity coverage, we decided to draw the sample from the luminosity-redshift distribution in 8 luminosity bins containing a similar number of clusters. The FoV criterion then calls for a staircase like distribution of these bins in the L_X-redshift diagram shown in the figure below. To obtain sufficient statistics, the minimum number of clusters in such a sample is of the order of 30. The affordable amount of XMM-Newton observing time for deep enough studies of a cluster does not allow for a much larger number of targets. Therefore we decided to plan for the selection of four clusters per luminosity bin. Full details of the sample selection can be found in Boehringer et al. (2007).


Page designed by Natasha Wood. Maintained by Judith Croston (e-mail: J.Croston AT soton.ac.uk). Last updated 7/4/2010