The REXCESS Large Project is a statistically unbiased sample of galaxy clusters selected from the REFLEX parent sample, observed with XMM-Newton. These were selected in luminosity-redshift space (Figure 1: for larger versions of all images, please open in new window), from which a homogeneous luminosity range was selected with no biasing on morphology. This sample was designed to fit inside the XMM-Newton field-of-view to assist with background statistics in the data analysis.


Figure 1. X-ray luminosity-redshift distribution of the REFLEX sample (small dots). The REXCESS sample are the data points selected in yellow boxes. The solid line is the survey flux limit, and the dashed lines are the distances at which R500 is 7, 9, 10, 12 arcmin respectively.

This cluster sample is relatively nearby (z < 0.2), which makes it an ideal sample for determining cluster statistics and scaling relations, which can be used for higher redshift Large Projects and cluster surveys.

This is a collaborative effort between many institutions and much of the work undertaken presently is to resolve differences between the different methods of data analysis. The work in Birmingham has recently revolved around two different types of analysis: morphology and mass.

  • Morphology. As this is an unbiased sample based on morphology it it provides an opportunity to study cluster irregularity and see if that correlates with other global gas properties of the cluster. A centroid shift method was used to quantify cluster asymmetry (Figures 2,3). A high morphology parameter implies a more irregular cluster than a system with a low morphology parameter.


    Figures 2 (left) and 3 (right): Morphological structure of RXCJ1302-0230. Blue circles are 0.3,0.5 R500. Contours are from smoothed X-ray image. The centroid shift of the cluster is indicated in the second figure off to the North-West.

    Cluster irregularity did not correlate with many cluster properties, with the exception of the entropy scaled by the empirically determined T^0.65 (Figure 4). The distribution of cool-core and non-cool core systems is indicated by the colour scale, and more irregular systems tended to have higher scaled entropy.

  • RFT_Fig4.jpg

    Figure 4: Scaled entropy (S/T^0.65) vs. Morphology. Red=non-cool-core clusters, Blue=cool-core clusters. Line=least squares best fit to data.

  • Mass.The X-ray total mass density can be derived under the assumption of hydrostatic equlibrium, and is dependent on radial temperature and density profiles. These were derived from an annular spectral analysis in XSPEC, with the appropriate profiles fitted parametrically. From this total mass profile, an NFW profile was fit to each cluster, to determine concentration parameters (Figure 5).


    Histogram of derived concentration parameters , c200. Red=non-cool-core clusters, Blue=cool-core clusters.

    The gas mass was subtracted off the total mass to leave mass density profiles that represented the stars and dark matter halo (Figure 6)


    Figure 6 (right): Dark matter + stars mass density profiles. Cluster irregularity is indicated by the colour bar (blue=more irregular). Line=NFW profile (arbitrary normalisation) using a concentration parameter of c200=4.

Researchers: R. Temple, A. Sanderson, T. Ponman (Birmingham)
External Researchers: H. Boehringer (PI), G. Pratt, A. Finoguenov (MPE, Garching), J. Croston (Hertfordshire), M. Arnaud (CEA,Saclay) + others in the collaboration