On the mass distribution in the Shapley Supercluster inferred from X-ray observations

Abstract

We present an analysis of a mosaic of ROSAT PSPC and Einstein Observatory IPC X-ray observations of 14 clusters and 2 groups of galaxies, enclosed in a sky area 15 degrees by 20 degrees centred on A3558 within the Shapley Supercluster region. From the mass of each cluster, extrapolated to a density contrast of 500, we define 4 large structures: (1) the core of the Shapley Supercluster with radius 13 $h_{50}^{-1}$ Mpc; (2) the core plus A1736; (3) the core, A1736 and the western extension (A3528-A3530-A3532) and A3571; and (4), adding the northern cluster pair A1631-A1644 to structure (3), the Supercluster as a whole enclosed within a radius $\sim 90h_{50}^{-1}$ Mpc. The observed total masses range between 3.5-8.5 $\times 10^{15}{M}_{\odot }$. The mass values derived from the observed intracluster gas (assuming a baryon fraction consistent with primordial nucleosynthesis) are of the order of few times $10^{16}{M}_{\odot }$. Given these estimates, the core is a bound structure with a very significant overdensity of at least 5 times the critical density, indicating that is approaching maximum expansion before collapsing. Structure (2) has an overdensity of 1.7 on a scale of $\sim 30h_{50}^{-1}$ Mpc. The core is then a 2--3.7 sigma fluctuation in an initial Gaussian perturbation field, normalized for galaxies and clusters. The highest value applies if we extrapolate to a density contrast of 200 and assume that ${\Omega }_b<0.095$. The baryon fraction of the core of the Shapley Supercluster, with A1736, is about 15 per cent over a radius of 28 $h_{50}^{-1}$ Mpc.

arXiv comment13 pages, 10 Postscript figures, LaTex, mn styler. MNRAS in press. Also available in postscipt format at http://www-xray.ast.cam.ac.uk/~settori/paper.html

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